Slight Risk For Central Gulf Wed

Day 1 Convective Outlook

NWS Storm Prediction Center Norman OK

0653 AM CST Wed Feb 05 2020



Valid 051300Z - 061200Z



...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TODAY AND TONIGHT

ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST...



...SUMMARY...

Severe thunderstorms are expected today and tonight across portions

of the lower Mississippi Valley and Southeast, with damaging wind

gusts and a few tornadoes.



...Synopsis...

The CONUS mid/upper-level pattern will be dominated by a major

synoptic trough -- initially extending from the Dakotas across CO to

northwestern MX -- and associated broadly cyclonic to southwesterly

flow to its east. The trough should move eastward gradually,

maintaining net positive tilt, as several shortwaves pivot through

it. By 12Z tomorrow, the trough should extend from the upper

Mississippi Valley across OK to north-central/central MX. Mid/upper

level southwesterlies (in the 500-250-mb layer) will intensify east

of the trough tonight, in a fetch from the TX Gulf Coast to the

upper Great Lakes.



Meanwhile, a strong but slow-moving surface cold front was analyzed

at 11Z from southern WV across eastern KY, becoming quasistationary

southwestward to a weak frontal-wave low between SHV-ESF, then a

cold front again across extreme southeast TX and the upper TX Coast,

to deep south TX. By 00Z, the low should ripple northeastward over

northern MS and western/middle TN, and the front should extend from

there to south-central LA and the northwestern Gulf. By 12Z, the

front should be located across eastern TN, central AL and

southeastern LA. A diffuse marine/warm front -- demarcating the

northern rim of best-modified boundary-layer air from the Gulf --

will spread inland across MS and much of AL, reaching southwestern

GA and the central FL Panhandle overnight.



...Southeast...

Episodic thunderstorm areas are forecast to move northeastward

across the region through tomorrow morning, offering the potential

for damaging winds and a few tornadoes.



At this time, the potential appears to be focused best in three

stages:

1. Prefrontal/warm-sector convection developing inland over MS and

into parts of northwestern AL during the day, as the air mass

gradually destabilizes from a combination of slow/diffuse diurnal

heating and boundary-layer theta-e advection. A persistent cone of

low-level confluent flow, with associated convergence amidst

weakening MLCINH, will support this activity. Mixed, potentially

messy storm modes are possible, along with some initially discrete

or embedded supercells, with flow aloft being largely parallel to

the zone of ascent.

2. Convection developing near the marine/warm front and farther

south into the optimally moist/high-theta-e Gulf warm sector this

afternoon into overnight. Coverage is very uncertain in this regime

due to the combination of weak CINH and weak forcing for ascent, but

some sustained/discrete storms are possible.

3. Near-frontal thunderstorms this afternoon through overnight --

initially over parts of LA/southeastern AR/MS and filling in

overnight as the frontal zone crosses the outlook area. Confidence

is greatest in relatively dense (scattered to numerous) storm

coverage in this regime, which also is likely to assume messy/quasi-

linear mode with time beneath nearly front-parallel flow. Still,

embedded supercells, bows and QLCS circulations will maximize severe

potential locally.

Convective coverage and longevity are still quite unclear,

especially in the first two regimes, given the "CAPE robber" stable

layer evident in midlevels in the 12Z LIX/JAN RAOBs, but not

upstream at LCH.



The warm sector will be characterized by increasing moisture/

buoyancy with southward extent toward the coast, favorable low-level

and deep shear area-wide, and stronger lift over northern and

frontal areas. Forecast soundings reasonably depict a broad area of

1000-2000 J/kg MLCAPE -- supported by surface dew points in the 60s

F, PW around 1.5 inches, and mean mixing ratios generally in the

13-15 g/kg range. Effective-shear magnitudes of 50-60 kt and

200-400 J/kg effective SRH should be rather common, which would

support tornado threats with any sustained/discrete cells, and

conditionally, significant-tornado potential given the parameter

space involved. Some subset of this broad risk area may be upgraded

once 12Z+ progs have arrived, and mesoscale trends become more

certain.



..Edwards/Kerr.. 02/05/2020



CLICK TO GET WUUS01 PTSDY1 PRODUCT

Excessive Rainfall Discussion

NWS Weather Prediction Center College Park MD

411 AM EST Wed Feb 05 2020



Day 1

Valid 12Z Wed Feb 05 2020 - 12Z Thu Feb 06 2020



...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF

THE SOUTHEAST & SOUTHERN APPALACHIANS...



...Southeast/Southern Appalachians...

A potent longwave trough continues to move east into the Plains,

establishing an atmospheric river in and near the Southeast with

moisture aloft from the eastern Pacific and Gulf moisture captured

in the low-levels. Divergence aloft to its east and southeast

will be favored. A warm front will lift north across the region

with another wave of moisture and instability surging northward.

Two axes of heavy rainfall are expected: one with an area of

low-level frontogenesis in and near Eastern TN as well as to the

south closer to the instability pool (MUCAPE values >1000 J/kg)

just inland of the Gulf Coast. Some elevated instability should be

able to sneak into the TN Valley. Precipitable water values are

expected to climb above 1.75 inches (which is 2.5-3.5 sigmas above

the mean). Inflow at 850 hPa increasing to ~65 kts, near the

magnitude of the mean 850-400 hPa wind, which would allow for

sufficient effective bulk shear for various modes of organized

convection where instability is > 500 J/kg (linear bands and

mesocyclones).



The overall synoptic pattern favors a broad area of heavy

rainfall, though recent two week dryness across much of the region

should keep the risk of excessive rainfall/flash flooding as

slight -- KY and northern TN has been a very recent exception due

to rainfall over the past 24 hours. Areal average precipitation

will be around 1.5-3" with locally higher amounts possible,

especially across portions of TN/AL and adjacent to the central

Gulf Coast region. The combination of moisture and instability

should allow for hourly rain totals to 2". Our model choice for

QPF was a blend of WPC continuity, the 00z National Blend of

Models, the 00z GFS, the 00z probability matched HREF mean, the

00z HRRR, the 00z ECMWF, and the 00z Canadian region model.

Adjustments made to the existing risk areas match recent changes

in WPC QPF.