River Flood Warning 9, NWS Huntsville, AL

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
829 AM CST Wed Feb 28 2018

.NEAR TERM...(Rest of Today)
Issued at 820 AM CST Wed Feb 28 2018

The first wave of rainfall associated with elevated theta-e advection
aloft has shifted into southern middle TN and far north AL. Rain has
ended from near Bear Creek to south of Guntersville. This trend
should hold for a few more hours with possible redevelopment later
this afternoon. We will be monitoring the model trends showing a
batch of dry air aloft/lower PWs that is advected into southern and
central MS/AL this afternoon and tonight. How far north this advects
into the TN valley will have a large impact on rainfall/flooding
considerations for our area by tonight. The hi-res short range
guidance indicates that redevelopment of showers and thunderstorms
will occur along the 850 mb warm front which will lie near the AL/TN
border. The NAM/NSSLWRF/localWRF all indicate this band to set up in
southwest into middle TN, just clipping our TN and far northwest AL
counties, as the warm sector lifts northward through the area. The
GFS is trending a bit more this way, but still has the band more
fully over north AL. For now, will only make near term PoP
adjustments to account for the current radar trends. This will lower
PoP today in our southern counties of north Alabama.

.SHORT TERM...(Tonight through Friday Night)
Issued at 300 AM CST Wed Feb 28 2018

The forecast challenge in the short term is where the boundary will
set up and therefore, where the axis of heavy rain will be. Outside
the discussion on the boundary, the other environmental factors for
heavy rain are in place, especially the PW values increasing to near
1.5 inches by 00z tonight and even a little higher overnight. In
perspective, the average for this time of year is around 0.5 inch
and 1.5 is near the daily max per the SPC sounding climatology.
Another and slightly stronger LLJ will be developing after 00-03z,
which will help to enhance the rainfall over the local area.

In terms of boundary placement, the GFS/ECMWf and the GEFS members
are in relatively good agreement on keeping the boundary and the
precip along the TN/AL border although the bullseye of the highest
QPF is in N MS, maybe far NW AL. GFS is forecasting almost 7 inches
of rain (probably a little TOO high) in NW AL with the ECMWF around
4 inches. The NAM remains the outlier with pushing the boundary north
this afternoon and then mostly dry until the cold front Thursday
afternoon. Even if the NAM verifies, it still gives the AL/TN border
around 3 inches which in this situation, would still cause some
flooding issues. Hires guidance is split but generally favors a
slight northern shift than this time the night before. One thing is
for sure, there is going to be a tight north to south QPF gradient
with areas around Cullman County and maybe even Dekalb seeing much
less rain.

Given the model consensus and WPC's excessive rainfall outlook, will
continue the forecast of the boundary right on the AL/TN border with
POPs increasing again to widespread by 00z Thursday through 12z. Then
the POPs were cut back in the eastern areas as the warm front lifts
north, giving a slight break in widespread rainfall. The cold front
will then follow and should be through NW AL around 15-18z and out of
NE Al by 00z Friday. With such a weak upper system though, the precip
with the front may linger a little past 00z in a few areas before
quickly clearing out with drier and cooler air in its wake. There
isn't much of an argument for thunder tonight through Thursday but
given the strength of the lift and the shear, can't rule out a few
storms and will keep the slight chance in the forecast. An additional
2-3 inches of rainfall are therefore possible tonight through
Thursday. Will not be making any changes to the Flood Watch with a
total of 2-4 inches possible, isolated higher amounts are likely.
Although incidences of Flash Flooding are certainly possible in
thunderstorms that occur, it looks like a few rivers are already
forecast to go into flood and areal flooding is also looking likely.

It will also be quite windy on Thursday with the combo of the
40-50kt LLJ and the tightening pressure gradient. Gusts up to 30mph
are certainly possible. Highs on Thursday will remain in the lower
60s with the warm advection but drop to the middle to upper 50s
Friday behind the front. Lows tonight remain mild around 60 but drop
drastically for Thu night with lows in the upper 30s and even lower
Friday night, near freezing.

.LONG TERM...(Saturday through Tuesday)
Issued at 438 AM CST Wed Feb 28 2018

Northwest flow aloft will exist across the local area this weekend,
as the region will remain to the east of an amplified 500-mb ridge
forecast to migrate slowly eastward across the MS Valley. At the
surface, the southern extent of a cold/dry arctic airmass across
central Canada will maintain a presence across much of the southern
CONUS, with light northeast flow and weak cold advection expected to
continue on Saturday--keeping temps in the m/u 50s. Conditions appear
favorable for strong radiational cooling on Saturday night, with a
light frost possible for most of the region. Temperatures will
rebound into the m60s on Sunday afternoon as the cold advection
subsides, although radiational cooling early Sunday evening should
allow temps to fall into u30s/l40s before the onset of stronger
southerly flow/cloud cover before sunrise.

Mid-level winds are forecast to back to the west-southwest and
strengthen considerably during the period from Monday-Tuesday as a
deepening northern stream trough closes off into a low and tracks
eastward into the Great Lakes. A cold front attached to the systems
occluding surface cyclone is expected to push southeastward across
the forecast area on Monday night, and although a few showers will be
possible as early as Monday afternoon the greatest coverage should
occur on Monday night. It appears as if the strongest synoptic scale
ascent related to the parent trough will bypass our region to the
north, and based on this as well the potential for very limited
northward return of moisture, we have lowered POPs into the high
chance range. Another cool/dry airmass will spread into the region in
the wake of the front on Tuesday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 518 AM CST Wed Feb 28 2018

CIGS will be tricky today and will likely fluctuate depending if it
is raining or not. KMSL just went to BKN008 once the rain stopped but
was hovering at BKN020 while it rained. It's back up again now that
rain started. The rain today will not be too widespread so CIGS may
sit in the IFR range for the majority of the time. Did bring CIGS
back to around 1000ft late today when more widespread rain will move
in which will continue through the end of the TAF period. VIS values
will also drop in rain, probably to 3SM or less, and rise when the
rain stops and CIGS drop.

Winds will be breezy today, around 10-15kt with an occasional gust to
20kts. These will diminish overnight and increase again by the end
of the TAF period.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Flood Watch through Thursday afternoon for ALZ001>010-016.

TN...Flood Watch through Thursday afternoon for TNZ076-096-097.

&&

NEAR TERM...17
SHORT TERM...LN
LONG TERM...70/DD
AVIATION...LN

For more information please visit our website
at weather.gov/huntsville.

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