Area forecast discussion
National Weather Service Huntsville Alabama
330 am CDT Tuesday may 21 2013
Synopsis...
latest water vapor satellite imagery showing a large upper level vortex
spinning over the northern plains (dakotas) with a smaller upper level
circulation moving across New Mexico...along the base of the main
vortex. Farther east...a narrow upper level ridge of high pressure
resides over the Tennessee/Ohio valleys but is getting squeezed between the
broad upper low over the northern plains and several weaker upper level
disturbances along the East Coast...between Florida and Virginia.
This primary upper level vortex which has been responsible for the
devastating tornadoes across OK/Texas the past few days...will
gradually migrate east over the next 24-48 hours...losing much of
its punch in the process but still setting the stage for a few rounds
of showers/thunderstorms across the Tennessee Valley late tonight through
Thursday. Latest regional radar mosaic showing well-defined line of
thunderstorms just crossing the MS river into western Tennessee/northwest MS. General
movement of the storms is N/NE...with a few discrete storms seen
developing ahead of the line over the past couple hours.
Djn.83
&&
Discussion...
warm and mainly conditions are expected today as the narrow upper
ridge holds one more day across the region. Broad area of convection
extending from East Texas to the upper Midwest/Great Lakes will
continue to drift east through the morning. Aforementioned line of
storms pushing into western Tennessee/NE MS has created a fairly pronounced
rain-cooled outflow boundary which should end up affecting portions
of northwest Alabama later this morning. This boundary could initiate new
convection as airmass heats up by midday. How far east this potential
convection GOES before interacting with upper ridge will be a challenge
today.
On the front side of the upper ridge...weak energy rotating counter-
clockwise around East Coast disturbances could touch off a few
rain showers/thunderstorms and rain across the southern Appalachians by early afternoon. Discounting the
overly aggressive 00z/21 NAM for today. Overall...believe much of
the area will remain dry today...however will mention threat of
isolated afternoon (10-20% pop) rain showers/thunderstorms and rain across northwest Alabama and NE Alabama/southern middle
Tennessee. If anything develops...not expecting convection to last long
beneath upper ridge.
Tonight into Wednesday...deeper moisture and energy arrive as
longwave upper trough finally approaches. Convection will likely
sustain itself along leading pre-frontal/850 mb boundary with maximum
low level convg and low level wind energy existing across northwest Alabama by
12z/Wed. Probability of precipitation overnight will be highest over northwest Alabama...with lesser
chances to the south and east until during the day Wednesday where
increased low level forcing /albeit weak/ along with a Theta-E maximum
will support scattered-numrs rain showers and a few thunderstorms and rain across the forecast area.
Marginal low level shear/instability and relatively weak middle level lapse
rates suggest a low potential for severe weather Wednesday at this time.
However...a few heavier showers/storms embedded in perhaps a weak
lewp could produce brief strong wind gusts late tonight through
wedensday afternoon.
Wednesday night/Thursday...shra/tsra are expected to diminish
Wednesday night but moist/unstable airmass coupled with passing upper
level waves could keep a few -shra going...especially across NE Alabama.
Surface front/leading edge of much drier air will extend across northern
Arkansas/northwest Tennessee/western Kentucky by 15z/Thursday. This boundary will acceleate
southeast and cross the forecast area Thursday night. Before this
happens...still influenced by broad longwave upper trough...additional
upper level energy within northwest flow aloft will March across the area
Thursday. This coupled with the approaching surface boundary could
support a brief round of rain showers/thunderstorms and rain Thursday afternoon. The 00z/21 NAM is the
most robust with this potential solution but even the medium range models
show something coming together.
Extended /Friday through Monday/...
much drier and cooler conditions will prevail Friday into Saturday
as high pressure builds across the eastern Continental U.S. In the wake of a
departing longwave trough. After a cool start Friday morning /lows in
the middle 50s/...temperatures should rebound to the middle to upper 70s beneath
plenty of sunshine. The coldest morning is expected to be Saturday as
lows could fall to the upper 40s in some valley locations. A "dirty"
ridge of high pressure over the central Continental U.S. Will gradually move
across the region by the end of the weekend into early next week.
With the area in a northwest flow aloft...the 00z/21 European model (ecmwf) is
slightly wetter and more aggressive with upper waves traversing the
down the frontside of the upper ridge. This is a new solution and relative
outlier with the other oper model/ensembles...so will lean toward a
drier solution Sunday/Monday but show increasing clouds. Temperatures will
rebound several degrees each day sun/Monday under the influence of upper
ridge.
Djn.83
&&
Aviation...
/issued 1216 am CDT Tuesday may 21 2013/
for 06z tafs...will continue in the near term with VFR conditions at
both hsv and mean sea level. Will keep in a tempo 5sm br group at mean sea level from 09z
to 13z to account for some fog that may develop. VFR conditions are
then expected through the rest of the forecast for hsv and mean sea level.
Tt
&&
Preliminary point temps/pops...
Huntsville 88 68 81 62 / 10 20 50 30
Shoals 87 67 81 62 / 20 30 50 20
Vinemont 85 65 78 59 / 10 20 40 30
Fayetteville 87 68 77 62 / 10 20 50 30
Albertville 84 68 80 61 / 10 10 40 40
Fort Payne 85 64 78 60 / 20 10 40 40
&&
Hun watches/warnings/advisories...
Alabama...none.
Tennessee...none.
&&
$$
For more information please visit our website
at weather.Gov/Huntsville.
