Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 051640

Area Forecast Discussion
National Weather Service Huntsville AL
1040 AM CST Mon Dec 5 2016

.NEAR TERM...(Rest of Today)
Issued at 1040 AM CST Mon Dec 5 2016

An area of showers moving across the Tennessee Valley as of this
writing, were part of a next round of precipitation set to affect
the region. This activity was preceding an upper low, that was
situated across western Mexico yesterday, now moving to the NE across
the Edwards Plateau of Texas. The trough axis as it moves ENE across
the Mississippi Delta will take on a neutral to slight negative tilt,
with it next moving across this region, then the Mid Atlantic coast
late Tuesday. This upper trough has induced surface cyclogenesis east
of the southern Texas coast. The low will track to the NE, moving
its center over to just north of our area late tonight and early

The approach of this system will bring widespread showers this
afternoon, continuing into the evening. Although some lightning
strikes earlier affected portions of eastern Mississippi and south of
us in central Alabama, will stay thunder-free into the afternoon
this area, given weak instabilities as we stay on the more stratiform
side of the system. This could change tonight as we get into a more
unstable regime.

Made only minor tweaks to high temperatures. Update was mainly to
remove earlier wording mentioning fog/drizzle.

.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

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 12Z TAFS through 12Z Tuesday morning)
Issued at 450 AM CST Mon Dec 5 2016

IFR conditions are expected across the area through 05/14Z with some
improvement to MVFR possible between 05/14Z-06/06Z as an upper level
disturbance sends a round of rainfall this morning. Then, later this
evening, showers are expected to increase in coverage with embedded
thunderstorms possible as a warm front moves north. Northeasterly
winds will occur prior to the approach of this warm front with gusty
easterly to southeasterly winds after 05/21Z. Included TSRA after
06/04Z when the threat is the highest as a surface low pressure
system crosses north MS/central TN. Lower IFR cigs are then expected
to return after 06/04Z.





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