Significant severe weather outbreak possible Saturday

Forecast Discussion: A strong shortwave is currently sweeping across the Four Corners region, which has induced the development of a surface low pressure system over the southeast Colorado/northeast New Mexico border. As the strong shortwave closes off into a mid-level low pressure system, the surface low will move into southwestern Kansas over the next few hours. A frontal boundary is currently stalled out across central Missouri, lower Illinois/Indiana and along the Ohio River in Ohio/Kentucky. A very moist environment exists south of the frontal boundary at and near the surface. As the surface low treks across Kansas tonight and tomorrow morning and into Missouri/Iowa late Saturday afternoon, the stalled front will begin moving northward as a warm front, replacing the dry airmass to its north with moisture-laden air. Throughout the day Saturday, surface dew points will increase across southeast Iowa and over the remainder of Illinois, Indiana, and Ohio.

surface analysis

Isentropic lift occurs when warmer air pushes up and over a colder airmass, which can generate precipitation. Isentropic lift, along with embedded weaker shortwaves, will continue to generate rainfall along and north of the soon-to-be warm front tonight into tomorrow morning across much of the lower Midwest. If you take a look at the Storm Prediction Center’s (SPC) severe weather risk area for Saturday, they have an enhanced and moderate risks extending from far northeast Missouri and southeast Iowa into northern/central Illinois, and northwestern Indiana. Please be aware that conditions in the morning (6-8am CT) may not feel like a severe weather day across the enhanced/moderate risk areas. Since the warm front will have not moved through much of the region at this point, temperatures will be somewhat chilly, especially with the rain falling. However, as the warm front surges northward, low-level moisture (humidity levels) will increase rapidly into the afternoon hours.

severe weather forecast

The surface low will move into western Iowa by mid-afternoon, strengthening further as it moves into the region. The region positioned just east of the surface low, ahead of the cold front, and south of the warm front will have the greatest risk of dangerous severe weather, as outlined in the SPC forecast. Rainy weather in the morning hours should move out quickly enough to allow for sufficient daytime heating at the surface. As the mid-level low approaches from the west, mid-level temperatures will cool, which will increase instability within the atmosphere. Surface-based instability allows air parcels (bubbles of air) near the surface and within the low levels of the atmosphere to begin rising. Surface heating and the addition of moisture makes those low-level air parcels buoyant. By cooling the mid-levels of the atmosphere, this ensures that the rising air parcels will remain warmer than the surrounding environment, allowing them to keep rising. This strong rising motion in the atmosphere on Saturday will result in deep thunderstorms developing across the risk zone, which will cause an increased risk for very large hail.

Tornado Risk: The enhanced and moderate risks also have been issued due to the tornado risk tomorrow. Within the treat zone, winds will flow from the southeast at the surface but will veer to the southwest with increasing height. We call this vertical wind shear. The position of the surface low relative to the mid-level low and shortwave will cause this turning of the winds with height. Given higher instability and vertical wind shear, tornadoes, some of which could be EF-2 or stronger, are expected. In fact, long-track tornadoes are possible. Keep in mind that the tornado risk extends southward into the Mississippi Valley ahead of the cold front as well; however, the tornado probabilities across that region will stay comparatively lower.

Chicago Tornado/Hail Risk: You might notice that the SPC only has Chicago under a slight risk; however, residents should watch the forecast closely tomorrow afternoon into the evening. The 18z NAM model is slower with moisture return across the Chicago area. By 5pm CT, the model projects dew points to only be in the mid to upper 40s. To the contrary, the HRRR model guidance has consistently projected dew points to be in the upper 50s or 60°F around the same time in downtown Chicago and well into the 60s in the western and southern metro. Timing matters a lot. If the low levels moisten sooner in Chicago, this could increase the tornado risk across the city, especially in the western and southern metro. Which model do we pick? That’s a tough question, especially when they’re trying to iron out intricate details. From what I can tell, the NAM model keeps conditions a bit cloudier across northern Illinois into the afternoon and even has storms moving through a little earlier than the HRRR model does. On the other hand, the HRRR has more daytime heating across the area, which would likely allow the warm front to advance northward more quickly. In effect, this would give developing and passing storms a much more unstable environment to work with. Honestly, I wouldn’t be surprised if the SPC bumps up the risk across areas closer to Chicago in their next update.

NAM model dew points
HRRR model dew points

Please continue to monitor the situation closely throughout the day tomorrow. These forecasts are never perfect, which means that you should expect some forecast modifications tonight and tomorrow morning.  

Active Pattern To Bring Anomalous Warmth East And Heavy Precipitation from Four Corners to New England

When a ridge of high pressure stubbornly sits over the same region, it generally brings dry weather and above average temperatures. A ridge causes sinking motion in the atmosphere, preventing deep clouds and precipitation from developing. However, along the edges of the ridge, or what I sometimes refer to as the periphery of the ridge, embedded disturbances in the flow will induce precipitation, some of which can be moderate to heavy. Notice the latest 72-hour rainfall totals (March 14-17 at 8am ET). Aside from localized convection, Florida and areas near the Gulf and Southeast coasts have remained dry through the period, due to sinking motion caused by the ridge. On the other hand, parts of the Southern Plains, Mississippi Valley, Tennessee Valley, Ohio Valley, and Mid-Atlantic have experienced wet conditions along the ridge’s periphery.   

Rainfall totals over the last 72 hours (March 14-17 at 8am ET)

Another hotspot for heavy precipitation over the last few days has been across California and other western regions. An upper-level low pressure system developed this past weekend and continues to slide southeastward along the West Coast. When a low becomes detached from the main flow, they tend to meander for a while and not move all that much. Eventually though, they get absorbed back into the main flow and make their way eastward/northeastward, which will happen by mid-week.

The evolution and track of the upper-level low will have an effect on the weather for the remainder of the week nationwide. As the feature approaches the Four Corners region on Wednesday, precipitation will develop and spread across the region. Mountainous regions across the Colorado Plateau have a high probability of picking up at least 4 inches of snow from Wednesday into Thursday, but accumulations will likely exceed at foot across the higher elevations. As the upper low moves northeastward into Colorado and then into the central Plains on Thursday, a surface low will develop just east of the Colorado Rockies. As the surface low moves across the central Plains and into the upper Midwest/Great Lakes region, a swath of accumulating snow will fall from Nebraska and the Dakotas into the far upper Midwest on Thursday going into early Friday.

The probability of snowfall accumulations exceeding 4 inches from late Wednesday into late Thursday

Also, the eastward progression of the upper low will amplify the ridge across the eastern U.S., which will drive up the temperatures substantially as the rest of the week progresses. Across the Southern Plains, Mid-South, Southeast, and Tennessee, temperatures will surge well into the 70s and into the 80s across parts of the area on Wednesday. The first round of moderate to heavy rain/storms associated with an embedded weaker disturbance will move across parts of the Southern Plains, Missouri Valley, Ohio Valley, and Kentucky/northern Tennessee on Wednesday. The rain will continue spreading northeastward into the Mid-Atlantic and lower half of New England later Wednesday into early Thursday. Some accumulating snow could fall across higher elevations regions in New England. The warm front will continue advancing northward on Thursday and Friday, expanding the warmth as far north as the Ohio Valley and Mid-Atlantic on Thursday and into the Great Lakes region/New England by late Thursday/early Friday. In fact, temperatures will probably increase overnight going into Friday across part of the Great Lakes region and western New England. Another round of rain/storms will move across similar regions in the Thursday/Friday timeframe ahead of an approaching cold front.

Green/yellow shaded regions have the high probability of having flash flooding on Wednesday or Wednesday night
Green/yellow shaded regions have the high probability of having flash flooding on Thursday or Thursday night

The cold front will sweep across most of the eastern U.S. by Friday/early Saturday, which will briefly usher in winter-like temperatures. Unfortunately, the cold front will take it sweet time fully moving through the Southeast; thus, expect Saturday to bring another day of 70s/80s across most of the Southeast outside of the Tennessee Valley and parts of the Mid-South. In fact, the cold front will likely stall out somewhere close to the Gulf Coast and northern Florida, so Florida and possibly surrounding regions just north may not experience much relief from the anomalous warmth.

Projected departure from average temperatures on Saturday morning

Hurricane Willa’s Remnants Will Have Large Impacts From Gulf States And Up The East Coast

Hurricane Willa is getting closer to the west-central coast of Mexico and should make landfall within the next few hours (Tuesday evening). Willa should retain major status until landfall. Willa will begin to weaken and lose its tropical characteristics as it moves across the higher terrain of Mexico into Texas but the remnants will remain well established to have large impacts from mid-week through the weekend for parts of the United States.

As the remnants move into Texas, deep moisture will stream northward throughout Texas into eastern New Mexico and Oklahoma. This will aid in heavy rainfall for this region with the greatest flood threat occurring in central and southern Texas, which has recently been inundated with rain. Widespread 1-3″ amounts are possible in eastern New Mexico and central and southern Texas with isolated 3-4″ amounts in central Texas near the Hill Country (see Fig. 1).

Fig. 1: 7-day rainfall forecast

The remnants will move out of Texas on Wednesday, merge with a shortwave, and begin to slowly intensify across the northern Gulf by Thursday. This will aid in thunderstorms and heavy rain for the Gulf States (from Louisiana to Florida) for late week. The low will then move off of the Southeast coast by late Friday and begin a north-northeastward forward movement off of the coast of the Carolinas. At this point, the low will begin to interact with an approaching cold front and deepen fairly quickly by Saturday morning as it spins off of the coast of the Mid-Atlantic. Heavy precipitation (see Fig. 1), rough seas and strong winds up to 30-60 mph (see Fig. 2)) will be possible for parts of the Mid-Atlantic and Northeast through the weekend. While coastal areas will see heavy rain (2-4″ are possible), areas further inland may see snow.

Fig. 2: Wind forecast Saturday evening

That is right, snow is possible as the nor’easter wraps in enough cold air for a transition to a heavy, wet snow. The best chance for snow will occur in interior parts of the Northeast down into the higher terrain of West Virginia. These areas may see a few inches of wet snow with several inches possible in the higher elevations of the Appalachians. Please keep in mind, we are a few days out so the snowfall forecast will likely need to be adjusted. A couple degrees cooler or warmer will have large impacts on accumulations and precipitation type.

Flooding is possible in Texas from this storm, which will impact travel. Turn around, don’t drown. Travel implications are also likely in parts of the Mid-Atlantic and Northeast due to wind and precipitation. With trees still having leaves on them, this increases the likelihood of them being overwhelmed for either wind or snow. This will increase the chances of power outages in this region over the weekend.

Big Cool Down On The Horizon

A decent cold front will move through central parts of the country late this week into the weekend. The sub-tropical ridge that has kept temperatures across the Southern Plains and Southeast above average recently will begin to move eastward as a trough moves in from the west. This will send a cold front south on Thursday and the front will continue its southward progression into the weekend. The front should move through Nebraska and into Kansas on Thursday into Friday, and through Oklahoma and northern Texas on Friday into Saturday. Tomorrow, along the cold front, a few severe thunderstorms look possible due to forecasted instability and shear values (see Fig. 1).

Fig. 1: Thunderstorm outlook map for Thursday

The front should stall across northern Texas and western parts of the Mississippi and Tennessee Valleys as it feels the influence of the ridge. This will keep temperatures warm for much of the Southeast outside of Arkansas, northern Louisiana, northwestern Mississippi and western Tennessee (cooler temperatures are in the forecast for other parts of the South/Southeast later in the extended period so keep reading for details on the cooler temperatures). Temperatures behind the cold front will be well below average for much of the Southern Plains and Midwest late this week and weekend (see Fig. 2 and 3).

Fig 2: Friday afternoon temperature anomalies

Fig 3: Saturday afternoon temperature anomalies

It should be noted, deep moisture will move into New Mexico, Texas and Oklahoma from Thursday into Saturday as the trough nears. This moisture will stream into this region from the Gulf of California where a Tropical Depression is located this afternoon. The increase in moisture will lead to increased rainfall, which may lead to flooding in parts of New Mexico, Texas and Oklahoma. Isolated areas in Texas and Oklahoma may see 2-6″ (see Fig. 4). The increase in precipitation and cloud-cover should keep afternoon temperatures below average even before the front moves through on Friday (see Fig. 2 and 3). High temperatures late this week into the weekend should be in the 60s and 70s for Oklahoma and 80s for most of Texas.

Fig. 4: Rainfall forecast through 7 days

Looking ahead to next week, a more amplified trough appears to usher in a reinforcing shot of cooler air. The cooler air will first be felt across the Northern Plains early next week, followed by the Southern Plains by mid-week, then eventually parts of the Southeast by late week into next weekend. Far Southeastern parts of the United States (Florida, eastern Georgia, South Carolina, and eastern North Carolina) may miss out on the coolest fall-like temperatures from this front but temperatures should still decrease. For other parts of the South, this will be the first significant cold front of the fall season. High temperatures will be well below average (see Fig. 5). It is too early to forecast high and lower temperatures with much confidence but right now it appears highs may be in the 60s and 70s with lows in the 40s and 50s behind this front. Locations further north will see temperatures much cooler than this.

Fig. 5: Temperature probabilities days 8 through 14

“Joyce” to impact Texas?

Firsthand Weather is keeping a very close eye on the convection located over the southeastern Gulf of Mexico and northwestern Caribbean (see Fig. 1). This area of convection will persist over the next 48 hours and should begin to organize by 48-72 hours into Tropical Depression/Storm Joyce. Right now, the upper-level winds are not conducive for organization but these winds will become more favorable by late week. The majority of the European ensemble members are showing development with a northwest movement into southern Texas by Friday (see Fig. 2) and the GFS and Canadian show a decent area of pressure falls (hinting at a Tropical Depression/Storm) in the same vicinity. This is why the National Hurricane Center (NHC) has given this area a 50% chance of development over the next two days and a 70% chance of development by day five (see Fig. 2).

Fig. 1: Convection that may develop into Joyce later this week

Fig. 2: European ensembles

Fig. 3: Area being monitored by the NHC
By late week, what could be Tropical Storm Joyce should be in the western Gulf. This will lead to copious amounts of moisture moving into south and coastal parts of Texas. Widespread 4-8″ of rainfall are possible with the possibility of amounts exceeding 10″ in areas (see Fig. 4). This part of Texas has received above average precipitation as of late, thus, the grounds are saturated; leading to an enhanced flash flood threat for the region from Thursday through the upcoming weekend.

Fig. 4: 7-day rainfall forecast
While a general WNW to NW motion is what is depicted by numerical guidance, much uncertainty does exist with the track of this system. Generally, the consensus of guidance has a landfall between Corpus Christi and South Padre Island. This could deviate further south or further north depending on the strength of the system. A stronger Tropical Storm/Hurricane would likely force a more WNW motion due to the 500mb ridge possibly intensifying to the north whereas a weaker Tropical Depression/Tropical Storm would likely track more to the NW.

Regardless of development into Tropical Storm Joyce, this system will lead to an influx of moisture into parts of southern Texas, which will increase the flood threat. Other hazards associated with this area of disturbed weather for Texas are: rough seas, rip currents and gusty winds. Keep checking back for updates because the waters are warm in this area so if organization occurs faster than expected, it is possible this system could ramp up quickly.

Gordon Intensifies, Voluntary Evacuations Issued

2:00PM Eastern Update (Monday):

Tropical Storm Gordon is a spinning off of the southwest coast of Florida. Gordon has intensified with maximum sustained winds of 50 mph and continues to have an improved structure on satellite and radar (see Fig. 1). The improved structure as favorable environmental conditions suggest further strengthening is likely and Gordon should become a hurricane over the next 24-36 hours before landfall across the central Gulf Coast.

Fig. 1: Latest IR imagery

A Hurricane Watch is in effect from the mouth of the Pearl River to the Alabama/Florida border (see Fig. 2). A Tropical Storm Warning is in place in Florida from Golden Beach to Bonita Beach, and for the Florida Keys from Craig Key to Ocean Reef, including Florida Bay as well as from the Okaloosa/Walton County line in Florida westward to east of Morgan City, Louisiana, including Lake Pontchartrain and Lake Maurepas (see Fig.2). Areas along the central and eastern Gulf Coast (including areas inland) need to prepare for heavy rainfall, gusty winds, isolated tornadoes, and rip currents and coastal flooding for immediate coastal areas within the next 24-48 hours. Excessive rainfall totals will be experienced well inland (see Fig. 3). It is possible these totals need to be increased further north into parts of Alabama, Mississippi, and Arkansas–this is heavily dependent upon the track of Gordon.

Fig. 2: Latest watches/warnings

Fig. 3: Precipitation forecast through 7 days

Gordon should make landfall Tuesday night along or near the Mississippi coast (see Fig. 4). Please note, anyone within the cone, and even locations near the cone, need to keep a close eye on the forecast over the next 24 hours. It is possible the forecasted track may change. This is a fluid situation and tropical storm conditions will begin to be experienced along northern parts of the Gulf coast by Tuesday. A voluntary evacuation order has been issued for Grand Isle, Louisiana and schools have also closed for September 4th in Grand Isle. People are urged to stay up to date along the coast in case local officials issue evacuation orders.

Fig. 4: NHC forecast cone

Tropical Troubles For Gulf?

Saturday Morning Update:
The NHC has given this tropical wave a 40% chance to develop over the next five days.

Friday Evening Update:
The tropics may begin to heat up “close to home” next week as a tropical wave eventually encounters more favorable conditions for intensification. Currently, the tropical wave is sitting south and east of Florida (near the island of Hispaniola) and has a 10% chance of development over the next five days according to the National Hurricane Center (see Fig. 1).

Fig. 1: NHC monitoring tropical wave
The tropical wave will move north and westward into the eastern Gulf of Mexico by early next week. At this point, the tropical wave may begin to organize and intensify into a tropical cyclone. Wind shear is currently preventing organization (see Fig. 2) but this should lessen by mid next week across the Gulf (see Fig. 3). Numerical guidance is struggling with the evolution and movement of the tropical wave. The European is currently stronger and shows the wave intensifying potentially into a tropical cyclone (see Fig. 4). The GFS, however, keeps this as an open wave. The strength will play a role in the movement of the wave. An open wave would likely track towards the northwestern Gulf whereas a tropical cyclone would likely move towards the north-central Gulf.

Fig. 2: Current wind shear (Friday)

Fig. 3: Future wind shear (Wednesday morning)

Fig. 4: European 850mb winds (Wednesday morning)
Regardless of intensity, tropical moisture will aid in heavy rainfall for much of the Gulf. Heavy rainfall and gusty winds will impact parts of Florida as soon as this holiday weekend followed by rain chances increasing for the northeast and north-central Gulf by early to mid next week. Several inches are possible across the northern Gulf next week (see Fig. 5).

Fig. 5: 7-day rainfall totals
This tropical wave needs to be closely monitored over the weekend. Updates on potential track and intensity will be provided as uncertainty decreases.

First Winter Weather Advisory Of The Season Issued In The Lower-48

Winter weather will grace parts of the northern Rockies early this upcoming week as snow levels drop to around 9,000 feet. The Billings National Weather Service (NWS) issued the first Winter Weather Advisory (WWA) of the season for parts of southern Montana (see Fig. 1).

Fig. 1: Current WWAs for Montana

2-7″ with isolated higher amounts are likely for the Beartooth and Absaroka Mountains (see Fig. 2). This will cause hazardous travel conditions where the heaviest snow falls. Snow will not be limited to Montana. Northwestern Wyoming and Northwestern Colorado will see snow in the higher elevations (see Fig. 3).

Fig. 2: Latest message from the NWS

Fig. 3: NAM snowfall forecast

Late-August snow is not unheard of for the northern Rockies. The jet-stream begins to dip slightly southward this time of the year, which can cause snow levels to drop. It’s just a matter of time before more areas in the lower-48 pickup their first snow of the season.

Western Relief (From Heat & Smoke) And Snow!

Much of the West has been plagued by heat and smoke but a change in the weather pattern will lead to cooler conditions and better air quality by as early as tomorrow (Thursday). A trough will build into the West through the end of this week into the weekend (see Fig. 1). This will allow cooler air to advect into the region. The greatest impacts will be felt across the Pacific Northwest (PNW) but impacts will not be limited to the Pacific Northwest–the Southwest will cool down as well.

Fig. 1: Trough building into the PNW

Temperatures will fall a good 15 to 30 degrees across parts of the PNW with more subtle cooling across parts of the Southwest but still allowing for below normal temperatures (see Fig. 2). For the PNW, high temperatures will be in the 60s to low 70s in the low elevations with higher elevations remaining in the 40s. There is even the possibility of snow for the highest elevations in the Cascades and northern Rockies where a few inches may fall. If you have plans to go up to Mount Rainer, snow will be likely from late week through the weekend. An increase in precipitation is likely for the region, too.

Fig. 2: Climate Prediction Center temperature probabilities (days 6-10)

Along withe the cooler temperatures and increase in precipitation chances, the air quality will improve as the trough builds in (see Fig. 3 and 4). It should be noted, this pattern change will change the direction of the surface winds, which will impact fire movement for wildfires that are not contained.

Fig. 3: Near-surface smoke (this morning)

Fig. 4: Near-surface smoke (Friday)

Hector To Impact Hawaii

Hurricane Hector appears it will move close enough to Hawaii to bring gusty winds and heavy rainfall from Tuesday night into early Thursday for parts of the islands (see Fig. 1). At this hour, Hector is a major hurricane (Category 4) with maximum sustained winds of 130 mph. As Hector continues its westward movement, it should remain a hurricane as it approaches the Big Island. This is due to the favorable environmental conditions including the anomalously warm waters in the region (see Fig. 2).

Fig. 1: National Hurricane Center Forecast For Hector

Fig. 2: Current Sea-Surface Temperature Anomalies (Tropical Tidbits)

While Hector is moving westward at this hour. A northerly component to its forward motion is expected early this upcoming week. This is because Hector is moving along the southern periphery of an upper-level high (forcing the westward motion) but the high should slowly weaken as a trough builds southward allowing the northerly component (see Fig. 3). How far north Hector will track is unknown. In the latest advisory, the National Hurricane Center believes the center of Hector will remain just south of the Big Island. With that said, it is possible Hector could make landfall in Hawaii. The average track errors are still close to 150 miles this far out.

Fig. 3: 500mb Geopotential Heights (Tropical Tidbits)

Regardless of landfall, Hector will cause tropical storm conditions for parts of the Big Island and possibly Maui, Moloka’i and O’ahu. Gusty winds of 30-45mph, heavy rain showers, and rough seas are likely. Please remain on high-alert if you’re in Hawaii or have plans to travel to Hawaii this week as just a small northerly jog could bring more severe impacts.