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

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Please note that this forecast was posted last Sunday; thus, this forecast is mostly outdated.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