Atmospheric River To Bring Heavy Rain and Mountain Snow To Pacific Northwest

Around this time of year, we have to closely monitor any atmospheric rivers that decide to make landfall along the West Coast. Under the right conditions, the influx of moisture brought in by an atmospheric river can result in a multi-day period of heavy rain and mountain snow. For those unfamiliar with atmospheric rivers, briefly read over this infographic from NOAA. Essentially, atmospheric rivers are narrow bands of anomalous water vapor content that are connected to the tropics.

North America from space. Elements of this image furnished by NASA

We can monitor these atmospheric rivers in real-time by using precipitable water imagery derived from microwave sensors on satellites. Higher precipitable water values equate to higher amounts of water vapor contained within the atmosphere over a given location. Basically, precipitation water maps tell us how much liquid water we’d have if the water vapor within the atmosphere above a given location at a given time were condensed into liquid. In the precipitable water loop, you’ll notice the tongue of high values that have already reached the Pacific Northwest. The next question becomes whether or not the atmosphere is currently primed to condense out a lot of that moisture from the atmosphere. If you’re in the Pacific Northwest, you probably already know the answer to that.

The Pacific Jetstream currently extends to the Pacific Northwest, with a jet streak just offshore. Basically, flight-level winds are currently anomalously strong over the region, and we monitor these winds because they indicate where surface lows might develop. Also, it’s important to note that these winds are mostly oriented west-to-east (zonal flow). In such a pattern, storm systems generally move in a zonal manner, so the Pacific Northwest will continue to capitalize on the injection of moisture from the atmospheric river.

A surface high pressure system currently sits offshore of California, shielding most of the state from any effects from the atmospheric river. In the Northern Hemisphere, wind flows clockwise around surface high pressure, and you can actually see in the precipitable water loop the atmospheric river moving along the periphery of the high pressure in a clockwise manner.

From tonight through New Year Day, basically the entire Pacific Northwest will be under the gun for heavy rain and mountain snow. We’re talking 4-6+ inch rainfall totals along and west of the Olympic Mountains and Cascades (as far south as most of Oregon) through tomorrow night. Along and just west of the Rockies in Idaho and Montana will also be at risk for higher rainfall totals over the period. Snowfall totals will easily exceed 1-2 feet around and above 4,000 feet through Wednesday night.

A shortwave ridge is going to begin building into the Pacific Northwest on Thursday and Friday, as a surface low treks toward British Columbia. In effect, the Pacific jet stream will become less zonal, and the atmospheric river will nudge farther northward into British Columbia near Vancouver Island or just to the north. Thus, the Pacific Northwest will temporarily become less active late week, although the western third of Washington State will still manage to get some precipitation during the period. If you want to see A LOT of snow, head to the Coast Mountains in British Columbia later this week.

By the weekend, the ridge will move out of the region, and the pattern will become active again across the Pacific Northwest. Surface high pressure will once again position itself offshore of California, directing the moist airmass away from the state toward the Pacific Northwest. In other words, the western third of Washington and Oregon will have to deal with more heavy precipitation this upcoming weekend.  

Late-Christmas week storm to impact a large part of the country

Firsthand Weather is tracking a potent storm system that will impact much of the Southwest late-Christmas week, with impacts spreading north & east into the Plains over the weekend followed by impacts across the East by early next week. A huge dip in the jet stream will develop by mid-week across the eastern Pacific with the development of a potent upper-level low.

Figure 1: Big dip in the jet stream by mid-week across west

The initial impacts will begin across southern California late-Christmas day. Heavy valley & coastal rain will fall, which may lead to flash flooding. Rain amounts up to 2″ are possible. In higher elevations, above 3,500′, heavy snow will fall. The high deserts may pickup a couple inches of snow with close to a foot possible in the higher elevations of the mountains in southern California. Rain & snow will continue into early Friday before shifting east.

Figure 2: Precipitation totals through Friday morning
Figure 3: Snow totals through Friday

As the upper-level low moves east on Thursday, it will spread its impacts into southern Utah, Arizona, New Mexico and southern Colorado by early Friday. Heavy precipitation is likely as this low taps into adequate Pacific moisture. Valley rain of 1″ is possible with higher totals possible east of the central mountains in New Mexico. The mountains & high deserts, above 5,000′, will see heavy snow. The heaviest snow looks to impact the southern mountains of New Mexico & Arizona. Over a foot of snow is possible above 7,000′ with a couple inches down to 5,500′. Rain & snow will continue for Arizona into Saturday before ending by late in the day with precipitation continuing across New Mexico & southern Colorado into Sunday morning.

Figure 4: Precipitation totals through Sunday
Figure 5: Snow totals through Sunday

The upper-level low ejects northeast on Saturday into the Plains. The low will begin to slightly weaken and may open up into a strong wave. There are still questions about the magnitude of the weakening, which throws a wrench into the forecast. Two other variables add to this confusion: the timing and track. All three of these will have significant impacts on the forecast. Regardless, showers & storm are likely for a large part of Texas, Oklahoma, Louisiana, Arkansas, Missouri, Kansas and Illinois on Saturday. It is possible on the northern shield of the rain & storms, snow will begin to mix in. The best snow hazard exists across the Texas Panhandle, northwestern Oklahoma and Kansas. It is possible snow may shift farther north into Nebraska & Iowa, if the system track farther north. Also, shower & storm chances will increase on Sunday for Mississippi, Tennessee, and Alabama as this system drags a cold front into the South. Rain totals of 0.50″-1.00″ look possible. It is too early to evaluate snow accumulations in this region, but an area of heavy snow (several inches) may setup in the aforementioned areas.

Figure 6: Dip in jet stream advances east into central parts of the country by the weekend
Figure 7: Precipitation totals through Monday for the Southern Plains
Figure 8: Precipitation totals through Monday for Plains & Midwest
Figure 9: Snow totals through Monday for Southern Plains

Sunday into early week, the precipitation will spread into the Northeast & Southeast. Cold air will be largely lacking across the Northeast but snow is possible for Maine with a mixture of wintry precipitation in Vermont, New Hampshire and eastern New York on Sunday. Shower & thunderstorm chances increase for Georgia and the Carolinas on Monday and continue throughout the day. Widespread 1-3″ of rain possible across the Southeast from late Sunday through early Tuesday.

Figure 10: Dip in jet stream moves to East Coast by early week
Figure 11: Precipitation totals through Tuesday morning for Southeast
Figure 12: Precipitation totals through Monday for Northeast

It should be noted: the graphics provided are strictly output from the European model to paint a general idea of the upcoming storm system & weather pattern. It is generally not recommended to use one model verbatim. The European does appear to be handing this pattern decently well, however, changes will be needed throughout the week so keep checking back for updates.

Weekly Regional Forecast – Dec. 15 – Dec. 22, 2019

This is an example of the weekly forecast that we provide for our subscribers every Sunday. This is one of many benefits that you get if you join the supporter group. The price is only $4.99 a month, and you can cancel anytime you’d like! Click here to join!

Please note that this forecast was posted last Sunday; thus, this forecast is mostly outdated.

Written by Matthew Holliday:

Arkansas, Louisiana, Tennessee, Mississippi, Alabama, Florida Panhandle, Georgia, South Carolina, North Carolina:

An incoming shortwave trough has begun the process of weakening the Southeast ridge, which has partially been responsible for the recent surge in temperatures. Low-level flow along the western periphery of a surface high centered off the Southeast coast has already been priming the environment for an active severe weather day on Monday. As the shortwave moves into the Southern Plains on Monday, surface low pressure will re-develop somewhere in the mid-South and then cross into the Tennessee Valley around Monday evening. The atmosphere will become increasingly unstable on Monday within the warm sector of the system, ahead of the cold front. Furthermore, vertical wind shear will become conducive for all modes of severe weather, including tornadoes, along a line extending from east-central Louisiana to central Mississippi (including Jackson) and central Alabama (including Birmingham).

Aside from the tornado risk, much of Tennessee, especially central Tennessee will be at risk for flash flooding (1-3+ inches). Enhanced lift will exist on Monday along and just north of a warm front associated with the low, which will put Tennessee at risk for flooding. As the surface low cuts across central Tennessee Monday night, strong and heavy storms ahead of the approaching cold front will add to rainfall totals across the area. Rainfall totals won’t be quite as high outside of the mountains across Georgia and the Carolinas. However, we still can’t rule out some localized flooding due to recent heavy rainfall in the area.

By Wednesday morning, the cold front will have cleared the entire area, which will bring in a colder and much drier airmass. Temperatures will drop into the 20s/30s across the entire region Wednesday night into Thursday morning before sunrise. In fact, there may be freezing temperatures down to the Florida panhandle. The colder air will hang on into northeast Georgia and much of the Carolinas due to cold air damming through late week or early weekend. An interesting scenario sets up this weekend with the potential development of a Southeast surface low. Uncertainty remains high due to differences in timing between the model guidance, but this is certainly a feature we will discuss more in the coming days.

Florida Peninsula:

A mid-level ridge currently extends westward across the Florida peninsula and is centered just off the Southeast coast. The ridge will remain far enough westward to mostly prevent thunderstorm development on Monday. With this early-weekly setup, temperatures will remain well-above average on Monday, with temperatures easily approaching 80°F across inland regions. The ridge will begin to weaken on Tuesday, as a shortwave trough sweeps through the Mississippi Valley and Tennessee Valley and induces the development of a surface low. Although this low will remain well-away from Florida, a cold front associated with the system will sweep across the entire state, temporarily pushing in a surge of cooler air. Thunderstorms could develop ahead of the front on Tuesday and Wednesday, but we don’t anticipate any widespread flooding with the front.

After the cold front passes through by mid-week, there will be a brief lull in the weather. Temperatures will drop into the 50s/60s behind the front on Wednesday, except across the southern tip of the state. By Thursday, the entire peninsula will experience temperatures below-average. In fact, temperatures should drop into the 30s/40s across the upper two-thirds of the peninsula by sunrise on Thursday. As an incoming shortwave from the Pacific will make it to the Southeast by the weekend, the weather could become very unsettled with the potential of heavy rainfall and storms across most of the state. Currently, there are disagreements in track and timing in the model guidance, making this aspect of the forecast highly uncertain.

Iowa, Minnesota, Wisconsin, Michigan:

The southern stream storm track has remained most suppressed farther south; which has allowed much of this region, with the exception of southern Iowa, to recently avoid additional snowfall. Temperatures will start out below-average early in the week, which will be further reinforced by a lobe of the tropospheric polar vortex dropping into the Great Lakes region by mid-week. Temperatures will drop well below zero across most of Minnesota and extend eastward into parts of Wisconsin by sunrise on Wednesday. The lake-effect snow machine will kick into high gear by early Wednesday and last into Thursday. Temperatures should begin to moderate significantly by the end of the week as a ridge moves into the region from the west.

Missouri, Illinois, Indiana, Ohio, Kentucky, West Virginia, western half of Pennsylvania:

Most of the region has had to deal with the first round of wintry precipitation today and tonight (Sunday). Get ready for round 2 on Monday and Tuesday. The driving mechanisms behind the second round of precipitation will be somewhat different than the first. The system responsible for the severe weather risk to the south will trek across Kentucky and West Virginia early Tuesday morning. Central Missouri/Illinois/Indiana/Ohio will add to today/tonight’s accumulations on Monday going into Tuesday morning. Accumulating snowfall will also occur across north-central Ohio and northwest/north Pennsylvania. Even though northern Kentucky has managed to get some snow with the first round of precipitation, the track of the low will ensure that regions south and along the Ohio River will only get rain for most of the second event. However, a transition to snow south of the river, especially in the mountains, should eventually occur on the backend of the system on Tuesday, as northerly/northwesterly flow advects colder air into the area. Similar to Tennessee, lift along and north of an advancing warm front will result in heavy rainfall totals (1-3 inches) occurring across central-eastern Kentucky and West Virginia before any transition to snow occurs.

After the early-week system pushes through, deep-layer northwesterly flow and the southeastward movement of cold, Canadian high pressure will ensure that temperatures are well-below average across the entire region from early Wednesday morning into Thursday. Ridging will build in from the west by the end of the week, which will result in a warming trend and slightly above-average temperatures. It appears that another potent shortwave will move into the eastern U.S. by the weekend; however, the resultant surface low may actually pass by to the south of this region.

Virginia, Maryland, Delaware, New Jersey, the eastern half of Pennsylvania:

Light snow accumulations will likely occur from northern West Virginia/Virginia, northern Maryland, and far southern Pennsylvania tonight through Monday morning. Low and mid-level warm air will advect into the region on Monday ahead of the approaching surface low that will move into the region from the Tennessee Valley and Ohio Valley. This warm air advection will ensure that snow transitions to freezing rain and then rain from south to north; thus, there will likely be freezing rain accumulations, as well, across the mentioned region. Elsewhere, it will be rainy along and ahead of the advancing warm front and then ahead of the approaching cold front on Tuesday.

Otherwise, the weather will be pretty uneventful across the entire region until the weekend. Deep northwesterly flow will set up across the entire area by early Thursday. Temperatures will be well-below average from mid-week until the end of the week, dropping into the single digits and teens across almost the entire region on Thursday morning. Temperatures will moderate some Thursday night into Friday, even though it will still be a bitterly cold night. Since cold air damming will occur late week into the early weekend, temperatures will be slow to moderate, despite a ridge that will begin to build into the region on Friday.

New York, Massachusetts, Connecticut, Rhode Island, New Hampshire, Vermont, Maine:

With half of December is now behind us, the Northeast is one of few areas in the U.S. where temperatures over these first two weeks have been consistently at or below average. A majority of the region will manage to either get some snow or a mixed bag of precipitation from the same system that will bring severe weather to parts of the South and wintry weather across parts of the Midwest and Ohio Valley. The surface low will move offshore by Tuesday afternoon and be positioned offshore near Cape Cod by Tuesday evening.

A warm nose just above the surface will works its way into New York City, Long Island, northern New Jersey, Connecticut, and Rhode Island early Tuesday. Thus, areas along the coast from Long Island to Cape Cod will have mostly rain, and areas inland just north and west of this region into Massachusetts/Connecticut will likely have a wintry mix. A fresh few inches of snow will likely fall across the remainder of New York state, Vermont, New Hampshire, and the southeastern half of Maine on Tuesday.

A piece of the tropospheric polar vortex is going to break off and briefly become displaced over the region by mid-week. First off, this will allow the lake-effect snow machine to kick into high gear on Wednesday. It’s even possible that a surface low will develop ahead of the vortex lobe, which may bring some scattered light to moderate snow across the Northeast. However, the big story will be the frigid air. Temperatures will dip well-below average and likely won’t recover much until mid to late weekend. On Wednesday nights, temperatures will likely dip near or below zero just south of the St. Lawrence River into the Adirondack Mountains in northern New York and into northern New Hampshire/Vermont and far western Maine. These bitterly cold temperatures will stick around through the remainder of the week. Temperatures won’t be too much warmer, even along and near the coast.

Written by Dr. Christopher Nunley:

West:

Short range: After an active pattern across much of the West, a quiet weather pattern will begin establishing itself across the Pacific Northwest and Southwest through the first half of the upcoming week. A large, dominating high pressure will move into interior parts of the Northwest. This will keep the first half of the upcoming week rather benign. Heading into the second half of the week, a more active pattern starts to take shape as a “split” pattern develops in the upper-levels of the atmosphere. This will ensure the Pacific Northwest & the Southwest both see active weather late week into the weekend. A nice surge of Pacific moisture will feed into the West Coast the second half of the week. This will generate an uptick in precipitation as lift increases with a series of upper-level disturbances that move across the region. Heavy precipitation can be expected from Washington south into northern California from Wednesday evening through Friday. Snow will fall in the Cascades, Olympics and Sierra-Nevadas while rain falls in the valleys & coastal areas. 2-5” of rain will fall with over a foot of snow possible in the highest parts of the aforementioned mountains—great news for ski resorts. Over the weekend, the precipitation focus shifts south into southern Oregon through California as the moisture continues to feed off of the Pacific ocean. With that said, Washington will still have lingering precipitation hanging around.

Long range: The weather pattern will remain active into the week of Christmas. A deep trough will establish itself over the eastern Pacific, which should aid in more disturbances moving onshore across the West. This deep trough should ensure the disturbances have colder air to work with, which will drop snow levels. The interior West looks to remain pretty benign until the week of Christmas. At this point, a potent storm system may move into the Southwest (southern California, Arizona, New Mexico) from the Pacific, which may have a nice moisture feed, aiding in heavy rain & snow for the Southwest and eventually transitioning north into Utah & Colorado.

Southern Plains & South:

Short range: Benign weather pattern for the Southern Plains & South after Monday. Temperatures will be cool & dewpoints will be low through mid-week before southwesterly flow aloft increases temperatures across the region. The next storm system to watch is late this upcoming week. A shortwave will dive southeast across the Rockies late week. With the southwesterly flow aloft increasing temperatures and moisture by late week, a few showers or storms may develop as this shortwave dives southeast across the Southern Plains. This shortwave should close off into an upper-level low over the weekend across the South. This would act to increase shower activity with a few storms possible depending on the moisture return and whether or not this system “cuts off” from the upper-level steering patterns. This will be ironed out over the next few days.

Long range: The benign pattern looks to continue into the week of Christmas with near average to above average temperatures & dry conditions. By the end of Christmas week, a storm system will move out of the Southwest. This will increase moisture chances across the Southern Plains and eventually the south. Too early for specifics, but there may be a thunderstorm threat with this system as well as a strong cold front moving in as the system departs. Snow looks possible across the northern part of the Southern Plains, which could impact travel as people are leaving their Christmas destinations.

Northern Plains:

Short range: Quiet weather expected for the Northern Plains as the well-established northwesterly flow that was setup across the region breaks down and a strong upper-level ridge amplifies over the region. This will prevent storm systems from impacting the Northern Plains and ensure temperatures warm to above average values.

Long range: The quiet weather pattern should hold into Christmas week. Toward the end of Christmas week, guidance is indicating the ridge will breakdown allowing a deep trough to move in, ushering in much colder air and allowing for widespread snow as a surface low sets up. The snow may be positioned closer to the Southern Plains. Too early to determine exact locations and impacts but this will be ironed out over the next several days.

High-impact storm to dump several inches of rain across the South and Southeast

A potent low will develop over the northern Gulf of Mexico late Friday night into early Saturday, which will aid in heavy rain from Texas east to the Carolinas through the weekend into early next week. The low will develop in response to an upper-level trough that is digging into Texas. This trough is expected to close off into an upper-level low late Friday. This will help allow the surface low to develop and quickly intensify.

While this low will not acquire tropical characteristics (the surface waters of the Gulf of Mexico are too cool right now), it is still expected to cause big impacts across the South & Southeast through early next week. The primary impact is heavy rain that may lead to isolated areas of flooding but a few other impacts are: severe thunderstorms & tornadoes, gusty winds & high seas that may lead to coastal flooding.

This low will pull in adequate Gulf moisture into the South & Southeast allowing for a large shield of moderate to heavy rain beginning late Saturday and continuing through Monday. A widespread 2-4″ is in the forecast with isolated 5-8″ amounts. Flash flooding is possible, especially for areas near and south of I-20 (Louisiana, Mississippi and Alabama) late Saturday into early Sunday with the flood hazard shifting east into Georgia and South Carolina late Sunday into Monday.

As the storm begins to get its act together late Saturday into Sunday, gusty winds are possible along the Gulf Coast from eastern Louisiana to the western coast of Florida. The elevated wind threat will eventually impact eastern Florida up through the Carolinas late Sunday into Monday as the low moves northeast. The counter-clockwise winds around this low may allow water to “pile” along the northern Gulf Coast that may lead to some salt-water flooding. Isolated severe thunderstorms are also possible along the Gulf Coast this weekend with the best chance on Sunday for the southern half of Florida. The main hazards are damaging winds but isolated tornadoes are possible. 

Let’s time out the onset and ending of precipitation. The graphics below are estimates from our numerical models in Central time.

This system is rather warm so the snow threat appears to be greatly limited in the southern Appalachians. The low will quickly race out to sea by early Tuesday, thus, impacts are not expected across the Northeast.

Tornadoes, damaging winds and large hail possible Monday across the South

Severe thunderstorms are likely for parts of the South Monday into early Tuesday. A potent shortwave will cross the Rockies late today and move into the Plains overnight Sunday. In response to this shortwave approaching the region from the west, warm & moist air will stream into the South Sunday night into Monday, helping set the stage for severe thunderstorms. As the shortwave treks east on Monday, it will send a cold front into the region. This cold front will provide the lift needed for severe storms to quickly develop.

The best chance for severe storms will be late in the day Monday. Initially, Monday morning, a few rain showers & drizzle possible across Louisiana, Mississippi & Alabama. A few non-severe storms will begin to develop along the cold front around 8:00 am across western Tennessee, northwestern Mississippi & northern Louisiana. Those storms will slowly intensify as the cold front moves southeast. By noon, a broken line of storms will exist along the cold front extending from central Tennessee toward the southwest extending into far southeastern Texas. The line of storms will begin to fill-in and intensify as the airmass destabilizes during the afternoon hours. A few storms may try to develop ahead of this main line across eastern Louisiana, Mississippi & western Alabama. These storms will have a favorable environment to produce high-impact severe weather. Heading into the overnight hours of Monday, storms will shift east toward Georgia and begin to lose some of their punch. (Note: use the graphics below as a tool for a general idea of timing/placement of thunderstorms. Exact timing and location of storms Monday will vary.)

The atmosphere will be primed for severe storms Monday afternoon & evening. There will be adequate shear and steep lapse rates. While morning cloud cover is expected across most of the South, as moist air feeds in from the South, a few breaks in the clouds can be expected by the early afternoon hours. This will allow for instability to increase throughout the day. These parameters will allow storms to produce all modes of severe weather; tornadoes, damaging winds and large hail are all possible. 

The best areas to see severe storms is across Louisiana, southeastern Arkansas, Mississippi and Alabama. But, the severe threat extends into eastern Texas, southern & central Tennessee, western Georgia and the Florida Panhandle. There is an enhanced risk of severe storms from central Louisiana east into west-central Alabama. This is where the best coverage of severe storms is expected and where the best chance for tornadoes will exist. 

The severe threat will continue beyond the daylight hours. Nighttime severe weather is particularly dangerous across the South. Make sure you have a plan in place in case a warning is issued for your area, and make sure you have reliable & accurate resources to receive all watch/warning information.

Freezing rain from northeast Georgia through Virginia

A complex forecast for parts of the Southeast & Mid-Atlantic tonight into Friday. A surface high that has been in place is beginning to move out of the region. The counter-clockwise flow around the high, as well as an approaching upper-level shortwave, will allow a rather quick return of Gulf moisture into the Southeast tonight and into the Mid-Atlantic tonight into Friday.

This moisture, paired with lift, will set the stage for precipitation to breakout. This is where the forecast gets interesting, a wedge of cold air will still be in place across parts of the Southeast. This will make precipitation types a tad tricky for far northeastern Georgia, northern parts of upstate South Carolina and North Carolina (southern Appalachians) tonight into early Friday morning. A brief window, an hour, of sleet is possible before a light freezing rain occurs for a few hours.

The freezing rain threat will shift north Friday morning into the southern Virginia as the precipitation shield advances north in response to a shortwave digging to the west. This will allow a surface low to develop, which will continue to spread moisture into the Southeast & Mid-Atlantic. Luckily, temperatures will begin to warm late-Friday morning leading to a changeover to rain for northeast Georgia, update South Carolina & most of North Carolina. The surface low will track northeast into the Northeast Friday night into Saturday causing a wet forecast for the Northeast over the weekend. 

Generally, most areas will see less than 0.10″ of ice, which is still enough to cause travel issues, but a few areas will pickup close to 0.25″. Whichever areas receive 0.25″, there could be icing issues on power lines & trees that may lead to a few power outages. The areas with the greatest chance of receiving 0.10-0.25″ are from western North Carolina north into western Virginia. Northeast Georgia & upstate South Carolina should see very minimal accumulations due to marginal surface temperatures and a short duration of freezing precipitation. Light snow accumulations are possible in the higher terrain of North Carolina & West Virginia. Due to the ice accumulation potential, Winter Weather Advisories are in place from North Carolina extending north through Virginia.

Wintry precipitation chances increasing for parts of the South and Southeast

Wintry precipitation chances increasing for parts of the South & Southeast. Numerical guidance continues to advance the Arctic cold front farther south & east faster than previous runs. The latest trends suggest a faster movement of an airmass that may be conducive for wintry precipitation as well as indications that the airmass will be one or two degrees cooler than initially thought. This is no surprise given how models struggle with these dense, shallow cold airmasses. This is increasing the opportunity for a brief window to see rain change to a rain/snow mix, possibly a complete changeover to all snow, late Tuesday into early Wednesday for parts of the region.

As the cold front moves through the mid-South Tuesday, followed by a passage overnight Tuesday into early Wednesday for the Southeast, impressive isentropic lift will occur in the post-frontal airmass due to a vigorous piece of energy to the southwest in the upper-levels of the atmosphere. At the same time, a nice feed of Pacific moisture will stream into the mid & upper-levels of the atmosphere. This will lead to a large shield of precipitation developing. Initially, based on forecast soundings, some of the precipitation may evaporate before reaching the ground as cold/dry air filters in at the surface behind the cold front (this evaporation process would act to allow temperatures at the surface to decrease a couple degrees). Once the lower levels of the atmosphere moistens, a cold rain would begin.

The big question is how long will the precipitation occur in the post-frontal airmass. Climatologically, 8 times out of 10, the moisture gets scoured out before the atmosphere becomes cold enough to support wintry precipitation; however, with the latest trends of the faster cold front arrival, the airmass being a couple degrees colder, and the Pacific moisture at the mid & upper-levels of the atmosphere (seeder-feeder), models are biting onto & converging on a solution that shows a rain/snow mixture will occur for central Arkansas, Tennessee, northern Mississippi, northern Alabama, northern Georgia and western North Carolina.

Figure 1: Future radar Tuesday evening
Figure 2: Future radar Tuesday night
Figure 3: Future radar Wednesday morning

Right now, no major accumulations are expected; however, if a transition to all snow occurs for a couple of hours, light accumulations would be possible–especially on elevated surfaces. The best chance for accumulations will occur in Tennessee where 1-2″ may fall. This forecast is extremely fluid and needs to monitored over the next 24-48 hours for possibly significant changes to the forecast.






			
				
			
			

Is the Southeast really going to get an ice storm in a week?

I want to take a moment to address the chatter about the possibility of an ice storm across parts of the Southeast next week. I’ll discuss how you should interpret model guidance, and why you shouldn’t take individual model runs verbatim. However, I’ll show you why the upcoming pattern favors wintry weather. But remember, providing specifics 7 to 10 days away from a potential event is beyond the capability of this science. Focus on the pattern instead!

I’ll give you a recent example of what not to do. Several of our followers have asked us about a post that has gone viral this evening. You always want to avoid focusing on individual model output a week in advance. If Atlanta received 1.5 inches of ice, that’d be catastrophic, but there are a lot of intricate details that could change this forecast many, many times between now and then. That’s why no one should ever make a post like this. It’s pseudo-science. I hope you view this more as a learning opportunity for our followers instead of a dig at someone’s credibility.

We’ve discussed on Firsthand Weather over these last few days (especially in our supporter group) why the pattern in a week or so could actually be promising for wintry precipitation across parts of the South. Of course, that’s assuming everything evolves as we’re expecting. We also noted that we had concerns about icing issues, which hasn’t changed.

You may have heard us use the term ‘split-flow pattern.’ Sometimes, a ridge will build across the Pacific Northwest, which can also extend into British Columbia and Alaska. This type of pattern often allows colder, higher-latitude air to spill into the U.S. east of the Rockies. Of course, the amplitude of the ridge and its orientation can affect how far south the colder airmass makes it. That’s only one component of the pattern. The other component is an active sub-tropical jet stream that sets up to the south of the western ridge, which provides a pathway for storm systems to trek across the southern states (sometimes from the West Coast to the East Coast). If you check out projected winds at the jet stream level in the latest GFS, notice how the Pacific jet stream splits before reaching the West Coast. Again, the northern component of that split help drives colder air into lower latitudes, while the southern component of the split helps transport moisture across southern regions.

I want our followers to look at all of this from a probabilistic standpoint. A split-flow pattern simply increases the odds of wintry weather occurring across parts of the South. Remember in the first Dumb and Dumber movie when Mary Swanson told Lloyd Christmas that there was a one in million chance that she’d go out with him, he was ecstatic. There was a chance!! At this point, there’s a chance, and for some of you, the odds are definitely better than one in a million.

In the coming days, we’ll need to begin identifying any trends in the model guidance, which is the second step. The first step is establishing how the pattern might evolve, which tells us what to potentially look for in future model runs. We’ll soon begin pinpointing regions that will be at risk for wintry weather, and if it becomes apparent that an event will unfold, only then will we begin forecasting amounts. Regions that run along, east and even south of the Appalachians should especially pay attention to any trends. If surface high pressure does trek eastward into New England and a surface low does move across the Southeast, it’s not unreasonable to suggest that an icing event could occur as far south as parts of the Carolinas and even northeast Georgia. There still remains uncertainty about the availability of sufficiently cold air elsewhere across the Southeast (e.g. the northern Gulf coast states). However, this upcoming pattern does provide better odds for wintry precipitation across parts of the South, which hasn’t been the case for most Decembers over the last decade. But don’t forget, focus on the pattern and trends, and don’t expect specific details this soon. We promise we’ll keep you in the loop on Firsthand Weather.

Make sure you join our supporter group on Facebook if you haven’t already. Click here to join. I post details there (e.g. my recent post made on Tuesday evening) that I’m currently not comfortable sharing publicly. You often see a finished project on Firsthand Weather. In the supporter group, you get to see more of the process. Please consider joining!