Looking way out, the SPC is highlighting a possible severe threat for the middle of next week....
Despite this variability within the guidance, model consensus exists regarding the presence of a moist and strongly sheared air mass from central TX eastward across the Southeast on D5/Tuesday, ahead of a cold front progressing southeastward. Thunderstorms are anticipated along the front, with the coverage and severity largely controlled by the proximity of the approaching shortwave trough. It seems prudent to leave the D5/Tuesday outlook area in place for this forecast, although a reduction in probability and/or area maybe in needed in subsequent outlooks if the slower trend currently progged by the 00 ECMWF appears most likely.
Depending on the timing of the shortwave trough, some severe threat may exist D6/Wednesday across the Southeast and D7/Thursday from the FL Panhandle into Mid-Atlantic. Dry and stable conditions are currently forecast for the CONUS on D8/Friday.
Last Edit: Mar 2, 2021 14:23:29 GMT -6 by SKYSUMMIT
This from this early morning's SPC discussion for the upcoming work week...
...DISCUSSION... Medium-range guidance is in good agreement that a modestly moist and unstable air mass will be in place from south TX northeastward across the Lower MS Valley and Southeast on D4/Tuesday. This air mass will exist ahead of deep upper low likely centered over northwest Mexico early D4/Tuesday. Guidance continues to struggle with consistency regarding the evolution of this upper low, with the GFS remaining the most progressive. Even with the timing differences, it appears probable that thunderstorms will develop ahead of this upper low along a cold front from the TX Hill Country through the Lower MS Valley. General expectation is for a line of storms to move from central TX eastward along the and south of the front through the lower MS Valley. Confidence in the timing of this thunderstorm development is low but consensus in the location of highest coverage and most probable severe area is high enough to maintain and expand the 15% delineation already in place.
Some severe threat may be realized across portions of the Southeast on D5/Wednesday and perhaps even D6/Thursda but low predictability resulting from the variance within the guidance regarding the progression of the upper low limits forecast confidence.
Dry and stable conditions in the wake of this upper low should preclude any severe threat for D7/Friday and D8/Saturday.
15% usually equals a Slight Risk. This would be Wednesday:
...DISCUSSION... Southern-stream upper low will likely be centered over far west TX early D4/Wednesday morning, with an associated surface low a bit farther east over the middle TX coast. This upper low is forecast to gradually move northeastward through the day D4/Wednesday, likely reaching the lower MS Valley by early D5/Thursday. Strong forcing for ascent will accompany this system and numerous thunderstorms are expected from across TX, the lower MS Valley, and the Southeast states. Much of this activity will likely be along and north of the front, but some potential for more warm-sector storms exists across southern LA and adjacent southern MS. Shear profiles in this region support supercells and enough model consensus exists to introduce a 15% probability over the area.
Severe threat could persist into D5/Thursday across GA as the upper low and attendant surface low continue eastward/northeastward. Most model guidance suggest dewpoints will be in the upper 60s Thursday as the surface low and cold front move through. Overall instability may be tempered by cloudiness and stunted daytime heating but there should still be enough instability for thunderstorms. As such, there is a risk for severe thunderstorms and forecast confidence is high enough to introduce a 15% probability area from eastern FL Panhandle/northern FL through southeast SC.
Dry and stable conditions are anticipated across the CONUS once this southern-stream system moves offshore late D5/Thursday.
Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0223 AM CST Mon Mar 02 2020
Valid 041200Z - 051200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER THE CENTRAL GULF COAST...
...SUMMARY... Severe thunderstorms are possible Wednesday afternoon through early Thursday morning along the central Gulf Coast.
...Synopsis and Discussion... Southern-stream upper low centered over far west TX early Wednesday is expected gradually weaken as it continues northeastward during the period. By early Thursday morning this low will likely be centered over the Mid-South region. As the low progresses northeastward, mid-level flow will increase downstream across the Southeast and Mid-Atlantic. The northern stream will remain active, with one shortwave trough moving off the northern Mid-Atlantic coast and another moving across the northern Plains into the Upper Midwest.
Surface pattern early Wednesday is forecast to feature a low over southeast TX, with a warm front extending eastward across southern LA to southern MS/AL. Air mass south of this front will be characterized by dewpoints in the upper 60s and modest instability. The severe thunderstorm threat Wednesday will be largely tied to the evolution of this surface low and downstream warm front.
A more progressive front will push the moist and unstable warm sector offshore before the surface low reaches LA Wednesday afternoon, limiting the overall severe threat to elevated thunderstorms north of the boundary. However, a less progressive front will keep the warm sector onshore, with the approaching surface low (and parent upper low) providing the lift needed for convective initiation. Veering low-level wind profiles and strong deep-layer vertical shear will result in a supercell wind profile and the threat for all severe hazards exists if the warm sector remains onshore. Current forecast is for the warm sector to be inland over southern LA, coastal MS/AL, and the far western FL Panhandle, where a Slight Risk has been delineated. Isolated hail is possible with the storms expected north of the front from east TX through southern AL.
Post by thermalwind on Mar 2, 2020 19:00:02 GMT -6
Toss a coin to your forecaster on duty this afternoon at LIX for a great AFD.
A bit to discuss coming up for the next few days, as we venture into a rather active weather pattern. The current state of the troposphere today indicates the northern Gulf coast within a prominent WAA pattern in the low-levels, capped by dry air in the H7 to H5 layer where winds stay zonal to the tropopause. To our north, a frontal boundary draped from northern Texas to the Ohio Valley region borders anomalously warm, moist Gulf airmass to the south with cold, continental polar air to the north. Along the cold front is a weak surface low, generally positioned across the AR/MO border. Our closer proximity to the weak surface low has kept winds generally between 180-190 this morning, but as the next kicker shortwave dives across the Great Lakes, the upper dynamics that support the aforementioned weak surface low will race northeast, which will steadily clock surface winds from the southwest through the evening and overnight hours. Associated with this kicker shortwave will be a reinforcing shot of cooler air, which will help nudge the cold front southeast to the northern Gulf coast come daybreak Tuesday. By this point, we will be positioned between the shortwave to our north, and an approaching strong upper-level low digging deep across northern Mexico which will support subtle mid-level ridging across the area. Regardless of (some) surface convergence with this front, there wont be much dynamic support for any widespread showers/storms. However, in such a rich Gulf moist airmass, it will not take much for some near coastal or marine shallow showers/drizzle to form. Blended guidance tries to hint at this possibility, but experience leads me to believe even weak surface convergence/WAA will be enough for some areas of drizzle - and have slightly bumped PoP`s between 18Z TUE and 00Z WED mainly along/S of I-10/12. Otherwise, warm temperatures and cloudy skies will prevail for Tuesday.
Late Tuesday night/Wednesday morning is when things become complicated, but interesting. The aforementioned strong ULL to our west advances closer. Downstream of the Upper Low across the NW GOMEX, accelerating 850mb winds will expand a large region of isentropic ascent above the surface stationary front draped generally along the MS/LA line or across the Florida Parishes. As upglide continues to increase due to increasing/northward surging 850mb winds and attendant low-level height falls, a widespread (expanding eastward) shield of precipitation will develop all across central/northern LA, to southern MS and southern AL. The zone along the surface front, northward to the apex of the 850mb LLJ (850mb front) across southern MS will be the main focus for the potential of elevated convection to develop. The environment aloft within this zone of low-level ascent contains some extremely impressive lapse rates (NAM3KM H7-H5LR -7.5 to -8C/km) with anomalously cold H5 temperatures of -13 to -15C (Even old school parameters including SWEAT nearing 550, TT in the mid 50`s). This may be concerning for some elevated storms dropping some large hail in the early morning hours on Wednesday. Threat zone will be from the I-10/12 corridor north across southern Mississippi where the SPC maintains a MRGL risk for the threats mentioned.
During the day on Wednesday, things become even more complicated, and even more interesting. As the 850MB front presses north, so will any attendant elevated convection (north across central MS/central AL). To our west, a developing surface low across central Texas will advance east but our area will generally enter a period of calmer weather (scattered showers primarily fueled by strong WAA). CAM`s here throw in a curve ball. With such a widespread mass of heavy rain to our north, large-scale cold pooling tries to keep the front from lifting north, regardless of the low trying to advance the front north as a warm front. This typically happens a lot in these types of setups, where there is a battle between the rain cooled airmass ahead of a warm front, and the approaching synoptic ascent associated with the surface low and trough. Either scenario paints a completely different picture of either widespread severe storms (as the cold pooling loses, warm front lifts north, and most of LA, some of southern MS enters the warm sector and winds back/veer supporting rotating cells) or a complete bust/rainy day (The cold pooling to our north presses the front south to the northern Gulf, the low rides the LA coastline along the front and all we are left with is rain and a few storms). Since we are talking about Day 3 - will ride with a split solution between the NAM3KM, GFS and EURO, with the front pushing north across the I-10/12 corridor. Just know, the better risk for severe weather will exist more to the south for coastal areas of SE LA, with less of a risk the more north. However, any sway north or south with the front will position more/less areas in a severe weather risk. For now, be advised this is a complicated (and interesting) set up that may likely change between now and tomorrow. Subsequent forecast updates will present an increase in confidence but for now, know things might change."
This is going to be a fun one for me to watch Wednesday.
Post by HarahanTim - Now in Covington! on Mar 3, 2020 15:18:10 GMT -6
Event: Flash Flood Watch Alert:
...Flash Flood Watch is in effect for tonight through Wednesday night for southern Mississippi and portions of southeast Louisiana...
.A nearly stationary frontal zone will remain draped over the Gulf States. An area of low pressure is expected to develop near the Texas coast early Wednesday and move eastward along this stalled front. This will set the stage for potentially heavy rainfall and possible flooding.
...FLASH FLOOD WATCH IN EFFECT FROM MIDNIGHT CST TONIGHT THROUGH LATE WEDNESDAY NIGHT... The National Weather Service in New Orleans has issued a
* Flash Flood Watch for portions of southeast Louisiana and Mississippi, including the following areas, in southeast Louisiana, Ascension, East Baton Rouge, East Feliciana, Iberville, Livingston, Northern Tangipahoa, Pointe Coupee, Southern Tangipahoa, St. Helena, St. Tammany, Washington, West Baton Rouge, and West Feliciana. In Mississippi, Amite, Hancock, Harrison, Jackson, Pearl River, Pike, Walthall, and Wilkinson.
* from midnight CST tonight through late Wednesday night
* Average rainfall amounts over a large area within the watch is likely to range from 2 to 4 inches. Rain rates may be close to 2 inches per hour at times.
* Impacts include flooding of low lying and flood prone areas; Rapid rises on area creeks and streams.
Instructions: A Flash Flood Watch means that conditions may develop that lead to flash flooding. Flash flooding is a very dangerous situation. You should monitor later forecasts and be prepared to take action should Flash Flood Warnings be issued. Target Area: Ascension East Baton Rouge East Feliciana Iberville Livingston Northern Tangipahoa Pointe Coupee Southern Tangipahoa St. Helena St. Tammany Washington West Baton Rouge West Feliciana
Post by thermalwind on Mar 3, 2020 16:19:59 GMT -6
Plenty of upglide to provide ample rainfall north of where our front sets up tomorrow. The exactly where will be challenging with all the colder outflow from the rain pushing toward the frontal boundary.
Our low going to ride the bottom side of that jet streak through here.
Along the best cyclonic vorticity advection from the upper trough tracking across Texas.
NAM showing the convective potential that might be available in the warm sector, but do note it doesn't really line it up well with the best of the dynamics on the NAM. I think LIX and SPC has the idea right with a pretty messy convective mode and locations depend on exactly where the front sets up.
The other note is there is some ridiculous shear showing up along the frontal boundary on our model soundings. Hopefully shouldn't get much surface based along that boundary but rotating storms in the blobs of storms that'll roll over shouldn't be surprising. Couple that with some healthy looking lapse rates aloft and the hail threat is definitely there where you get anything rotating. Over an 1" certainly on the table for some locations. Dry slot at 700 mb and above presents a slight straight line wind risk too, but the winds aren't really rocking at those levels so it doesn't seem like wind damage would be super wide spread. Just worth being prepared for.
Finally the tornado risk looks very isolated and likely to be short lived with a messier convective mode. Anywhere below the warm front and along it just needs to remember the potential is there and take warnings seriously. There is potential there though for stronger (EF2+) tornadoes to happen if everything lines up right, so again, be aware of warnings.
Will do a in situ look in the morning to see what the soundings, upper air maps, and CAMs are trying to tell us.
Post by thermalwind on Mar 3, 2020 16:35:16 GMT -6
And once again, an excellent AFD from LIX this afternoon.
There is a
significant difference in model guidance - the GFS solution pretty
close to the previously desribed scenario, the ECMWF farther
south. The ECMWF solution would maintain the warm sector closer to
and off the coast, but placing the stronger isentropic lift region
over the forecast area.
The GFS would place a rich warm sector
over the area generally between 04/21Z - 05/09Z. One has to
consider the role of marine layer stability as near water
temperatures are still in the mid to upper 50s. Strong dynamics
can overcome this aspect, so deep surface based convection needs
to be expected.
Latest CHAP output using the 12Z GFS and NAM is
hitting hard on severe parameters. Ricks Index values ranging
160-177 which is around 65% chance severe. CHAP values: EF-2/3
tornado risk, 1.5-2.0 inch hail diameters and gust potential 70 to
80 mph. Additionally, frontal focusing will amplify rainfall rates
and efficiency for the potential of heavy rainfall and possible
flooding. CHAP precip is quite bold in some locations, with rates
indicated in the 2-3 inch/hr range.
The Euro solution means more rain as we get under the frontal lifting. Warm sector stays offshore.
The GFS on the other hand has most of this forum under the warm sector. While I don't expect widespread severe, there's absolutely some higher end potential with storms that do get to severe levels.
So yeah, we're going to be looking at the surface obs hard tomorrow because where the warm front sets up is central to who's got that severe risk to be worried about.
Guidance amounts are not as
high for storm totals, but rates will be the main concern,
particuarly as features coalesce along and into the stationary
boundary. As a result, a Flash Flood Watch has been posted in
collaboration with neighboring offices for all of southern
Mississippi and the Florida Parishes of southeast Louisiana
tonight through about sunrise Thursday. Storm totals of 2 to 4
inches is expected within the watch area, but some locations may
get this in 1-2 hours time at peak rainfall. Outside the watch
area, rainfall averages are indicated to be 1 inch or less as
marine layer influences may limit duration and extent of rainfall
along with lack of focus away from the front.
Just an idea on the expected rainfall.
Next consideration will
be the potential for wake depression low isallobaric wind response
late Wednesday night into Thursday morning. This would occur well
after any precipitation has ended and prior to the upper trough
passing through. A period of 1-2 hours may see a corridor of
enhanced winds in the 40 to 60 mph range, perhaps higher. If such
a phenomenon is observed, it would warrant a High Wind Warning,
but that won`t be issued until observed or imminent.Cold front ultimately
moves through by very early Thursday morning to flush the inclement weather out
and cooler high pressure builds into the region through Thursday.
This bit is certainly worth noting. Remember how the winds ripped after that squall line in April a few years ago that knocked the train over in NOLA? Same sort of idea. Potentially could have some very strong winds after the rain passes through. This would be overnight too. Would potentially take a lot of people by surprise.