Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 050828
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
228 AM CST Mon Dec 5 2016

.NEAR TERM...(Today)
Issued at 227 AM CST Mon Dec 5 2016

A cool, foggy, and drizzly day will prevail early this morning
before a negatively tilted shortwave trough that has caused
widespread heavy rainfall over the gulf coast moves northeast later
today. As it moves northeast the LLJ that is currently oriented from
the southwest will gradually back to the southeast late this
afternoon. Before that happens though, synoptic scale ascent will
gradually begin to shift northeast this morning resulting in the
development of light to moderate shower activity originating from the
lower MS river valley and gulf coast region. Have created a gradient
in POPs through the day today from south (highest) to north (lowest)
to capture the increase in convective activity today. May make some
last minute adjustments with the rain forecast depending on the
latest precip (or no precip) trends. Temperatures should gradually
rise into the low to mid 50s (i.e. little change from where
temperatures began this morning). The amount of precipitation
anticipated for this morning should be on the light side--generally
<0.20 inch. The highest rainfall amounts will occur later this
afternoon and through the overnight hours tonight.

.SHORT TERM...(Tonight through Wednesday night)
Issued at 227 AM CST Mon Dec 5 2016

As mentioned above with the LLJ, a WAA profile is expected to develop
late this afternoon as an occluded sfc low pressure system moves from
northeast LA to central MS and a negatively tilted shortwave trough
shifts northeast. This will send a `warm` front towards the TN valley
late this evening. This `warm front` and -divQ (convergence) and
instability/shear environment may cause the development of discrete
strong to severe cells over central and southern AL/MS. Over the TN
Valley, the instability profile is weaker but deep layer shear is
strong enough to support a marginal severe threat mainly over the
southern tier of the HUN CWA. The primary threat would be gusty winds
up to 50MPH with a low potential for damaging winds. The main
limiting factors for severe weather over northern AL is the elevated
nature of the instability overunning cooler low- levels, a cold air
damming/wedge front, and the low amount of instability in general
(50-100 J/kg). As the surface low pressure system crosses just to the
northwest over northern MS/central TN between 03Z-09Z there could be
an additional brief threat for weak rotation within convective
portions of the line. However, with the lack of sb instability
expecting the rotation to remain elevated. One concern is that this
could force momentum downward creating threat for gusty to damaging
winds.

As the parent negative tilted trough and occluded surface front
moves northeast, widespread convective showers will continue through
the overnight with embedded thunderstorms. Rainfall amounts will
approach 0.50-1.50 inches across the TN Valley with the highest
amounts near any embedded thunderstorms, and when the surface low
crosses northwest AL between 00Z-06Z (6PM-midnight). Seeing that the
models are depicting a convective band moving across between the 03Z-
09Z time frame which could mean even higher precip amounts (closer to
1.50-2.00 inches). Will trend QPF higher to account for this
possibility. Temperatures are going to likely hold steady if not rise
overnight so that the lows/highs for the day may occur at different
times of day.

Then, on Tuesday have kept a gradual decrease in shower activity
during the morning and afternoon hours with precip ending by late
afternoon. After precip ends, on Tuesday night there will likely be
prevailing broken cloud cover and light winds. However, patchy fog
(and possibly dense fog) may develop on given the saturated soils and
saturated low-levels of the atmosphere. So, have included patchy
fog in the forecast, but if the cloud cover actually decreases
further, more substantial fog could be possible. Near 30-year
climatological normal daytime high temperatures (in the 50s) and
mostly cloudy conditions should continue on Wednesday. Meanwhile, a
broad trough approaching from the central Plains is expected to move
towards the Southeast pushing an Arctic front and bringing a cold
snap for this weekend. Light rain is expected to accompany the front
on Wednesday night with temperatures quickly dropping behind the
frontal passage on early Thursday morning.

.LONG TERM...(Thursday through Sunday)
Issued at 227 AM CST Mon Dec 5 2016

Arctic front will have passed through the area by Thursday morning
continuing a trek south to the Gulf Coast by afternoon. Lingering
light rain is possible, mainly through midday. The lower atmosphere
will be drying out quickly though upon the entry of the cA airmass.
This may negate the possibility of any -sn before the colder air
arrives, so opted to keep it out of the forecast. The high pressure
center will track southeast into the TN valley on Friday. After lows
in the upper teens and lower 20s, highs Friday will only reach in the
middle 30s in valleys, with the Cumberland Plateau in the upper 20s
to lower 30s. Saturday morning should be the coldest with lows in the
teens to around 20. As the high drifts to the Atlantic Coast,
southerly flow and warm advection should develop Saturday into Sunday
as mid to upper flow becomes more zonal. We will introduce rain
chances Sunday Night with shortwave/cold front approaching of Pacific
origin. Low level moisture transport will increase greatly Sunday
Night as a southwesterly low level jet of 40-50kt is progged to
develop. The zonal synoptic pattern will likely play havoc with
timing and evolution forecasts in coming days.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1045 PM CST Sun Dec 4 2016

8H wave axis in now over north central AL. Ceilings over KMSL have
risen to arnd 1200 ft while KHSV is ovc at 400 ft with -dz. As the 8h
wave moves out of the cwa (arnd 10z), the chc of -dz should begin to
taper off from w to e across the TN valley. However patchy fog will
still be possible especially over KHSV until 14z. There could be a
break in the chc of pcpn between 14z to 20z. More shra are then expected
to develop after 20z as a closed upper low approaches the area. The
coverage and intensity of shra will increase after 00z across the taf
sites. For now will keep all pcpn as shra since new NAM data brings
in the higher/best instabilities after 09z (early Tuesday mrng).
Otherwise mainly ifr conditions are expected thru 14z especially at
KHSV. After 14z mvfr conditions are expected at both taf sites.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...SL.77
SHORT TERM...SL.77
LONG TERM...17
AVIATION...007


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