FXUS64 KHUN 182350

Area Forecast Discussion
National Weather Service Huntsville AL
550 PM CST Mon Feb 18 2019

For 00Z TAFS.


.NEAR TERM...(Tonight)
Issued at 316 PM CST Mon Feb 18 2019

The thick cirrus deck was able to thin through the day allowing for
some sun to peak through. This has resulted in temps in the mid to
upper 40s with a few sites reporting 50 degrees. As we go through the
night even though the high pressure to our begins to retreat to our
north, the developing surface low over TX should continue to support
winds out of the north. This will advect some cooler air into the
region overnight with temps falling into the low to mid 30s. If
clouds increase more than forecast temps may not fall into the lower
30s. The frontal boundary situated along the gulf coast will begin to
lift back as a warm front closer to sunrise on Tuesday. Drier air in
the lower levels may keep some of the initial precip as virga.

.SHORT TERM...(Tuesday through Thursday)
Issued at 316 PM CST Mon Feb 18 2019

Models have continued the trend of shifting the heavier rain band to
our W/NW on Tuesday. This is in response to a deeper trough across
the western US, slightly stronger ridge to our SW, and a CAD event in
the Lee of the Appalachians. These factors will take make of the
forcing and focus for heavier rains on Tuesday along and west of the
MS River. This more westward shift in latest guidance has lowered
rainfall totals from past forecasts. However, any shift in where this
axis sets up will have a big impact on rainfall totals.

As the trough moves out of the desert SW and into the Plains Tuesday
into Wednesday it begins to shift the heavier precip axis associated
with the cold front eastward. Moisture transport ahead of this boundary
will be very high with PWAT values approaching records for February.
Couple the influx in near record moisture with a strong 850mb jet,
the right entrance region of the 250mb jet over the area, and strong
isentropic ascent and you get strong vertical motions situated along
the cold front. Since the front will be delayed until after sunrise
Wednesday it is looking increasingly possible that we'll have enough
time for CAD to weaken and winds to become more southerly. This will
advect better low level moisture into the area and with steep lapse
rates aloft, which would support thunderstorms to be embedded in the
axis of heavy rainfall. MUCAPE from the GFS could range from 200-500
J/kg and when combined with a veering wind profile, there is a risk
for some stronger thunderstorms Wednesday afternoon.

The biggest concern is that if the frontal boundary stalls out over
the TN Valley for a longer period on Wednesday, any thunderstorms
that develop could really enhance rainfall totals. Rain rates could
be upwards of an inch or two per hour, which could start to overwhelm
drainage basins. The current forecast for the 6am to 6 pm Wed period
calls for between 0.5 to 1 inch. So, if thunderstorms do form and
there is training of these storms these totals could be very

After coordinating with TVA, there is large amount of water in their
upstream reservoirs and they are forced to release much of that
water downstream. In doing so it would raise river levels into Minor
flood stage by Wednesday. And that is just a result of water being
released downstream, not truly taking into account the added rainfall
from this event. So we could be looking at a situation where water
along many of our drainage basins won't have a place to drain to and
water just begins to backup. Based on this weekends event and the
responses we saw in river/stream levels, soils are likely completely
saturated and can't hold any additional rainfall. So, if the heavier
rain axis sits over the area for any length of time on Wednesday, all
of the rain will be runoff and could result in some significant
issues. As the previous forecast stated, the amount of rain on
Wednesday will be heavily dependent on the duration the band sits
over the area.

By Wednesday night into Thursday morning, high pressure moving into
the OH Valley will push the cold front to our south. This should give
much of the area a brief reprieve from the moderate to heavy rain.
Some models suggest we could see some lingering light rain Thursday
due some weaker shortwaves moving through the upper level
southwesterly flow. Starting Thursday afternoon into the evening
hours the frontal boundary to our south will begin to lift back
northward as a warm front. We should begin to see a return of the
moderate to heavy rainfall overnight as this boundary crosses back
over the area.

.LONG TERM...(Thursday night through Sunday)
Issued at 316 PM CST Mon Feb 18 2019

Extended range guidance from a consensus of the global scale
deterministic and ensemble models suggests that a subtropical ridge
(initially centered east of the Bahamas) will retrograde and weaken
by Sunday, while a broad positively-sheared longwave trough remains
intact across western North America. The net result will be a
continuation of seasonably strong/deep southwest flow across the TN
Valley for the majority of the forecast period. Within this regime, a
mid-level vort max located off the Pacific coast of Mexico is
predicted to eject rapidly northeastward in the southern branch of
the jet, but should shear apart in increasingly confluent flow aloft
before reaching the mid-MS Valley around 12Z Saturday. Regardless of
the weakening nature of this disturbance, an axis of surface pressure
falls related to its movement will slowly become established to the
north/west of the TN Valley late this week, allowing an effective
warm front to surge northward through the region on Friday afternoon.
Widespread light rain is anticipated to redevelop in the cool sector
to the immediate north of the retreating warm front late Thursday
night and continue through Friday morning, with a low probability for
thunderstorms expected to expand northward throughout the day as the
increasingly unstable warm sector builds northward.

Scattered-numerous showers and a few thunderstorms will continue
across the region from late Friday night and throughout the day on
Saturday, although with a general lack of synoptic scale forcing for
ascent over the warm sector we have indicated a gradient in POPs with
highest values over the northern/western zones. Meanwhile, a
stronger mid-level shortwave trough embedded in the broader western
CONUS longwave is expected to lift rapidly northeastward from the
southern Rockies into the eastern Great Lakes on Saturday-Sunday.
Low-level southerly flow will increase considerably on Saturday in
response to the movement of the stronger shortwave, with highs
forecast to reach the l-m 70s assuming a decreasing trend in
precipitation and at least some breaks in cloud cover. As the
deepening surface low related to this disturbance lifts northeastward
across the central Plains and into the mid-MS Valley on Saturday
afternoon/night, the trailing cold front will begin to accelerate
eastward, initiating a linear convective feature across western
portions of the lower MS Valley on Saturday afternoon which will
impact our region late Saturday night/early Sunday morning. Both
deep-layer and low-level shear will be more than adequate to support
storm organization, and with dewpoints in the l-m 60s supporting
MLCAPE potentially as high as 500-1000 J/kg, we are certainly
concerned about the potential for a severe thunderstorm event.
Canadian surface high pressure will build southward into the region
in the wake of the cold front, with cooler and drier conditions
anticipated on Sunday.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 550 PM CST Mon Feb 18 2019

Cold high pressure situated over the Northern Plains will build
eastward, resulting in NE winds ~10kt becoming easterly overnight and
on Tue. A more ESE component above the surface will bring additional
moisture preceding a developing area of low pressure over the NW Gulf.
This system moving NE will bring lower clouds and begin rain chances
across the TN Valley, from south to north during the Tue. Easterly
winds should increase into the 10-20kt range later Tue afternoon, as
a pressure gradient around the developing low strengthens. MVFR
CIG/VIS reductions are possible early Tue afternoon, with IFR
conditions developing late in the TAF.


AL...Flood Watch from Tuesday morning through late Thursday night for

TN...Flood Watch from Tuesday morning through late Thursday night for



NEAR TERM...Stumpf

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