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Area forecast discussion...updated 
National Weather Service Shreveport la
719 am CDT Tuesday Apr 21 2015

Update...
zone update just sent to add slight chance probability of precipitation areawide to
account for weak ripples in the wrly flow contributing to the
development of unauthorized -shra over deep East Texas now entering
west-central la. A step outside reveals that elevated instability is
present as evident by the altocumulus castellanus towers to the S of the office over
Toledo Bend. Thus...can/T rule out isolated thunder especially as
the middle level shortwave over southern OK/North Texas approaches from the west.
Quantitative precipitation forecast amounts though will be very light though given the very dry
low level air mass in place.

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&&

Previous discussion... /issued 623 am CDT Tuesday Apr 21 2015/

Aviation...
VFR conditions across the region will continue through at least
22/06z. Middle level altocu decks will move east across most of region
this morning...with general scattered decks to sky clear by later in afternoon.
Airmass to become increasingly unstable this afternoon...but likely too
dry for convection. Weak southerly flow in NE Texas may yield a few
MVFR decks after 22/06z into klfk...and possibly ktyr...kggg...and
kshv after 22/09z. Patchy fog is possible areawide by around
22/09z as well as weak surface based inversion develops. Deeper
convection possible Wednesday afternoon...beyond end of this forecast
cycle. /Vii/

Previous discussion... /issued 524 am CDT Tuesday Apr 21 2015/

Discussion...
surface high pressure has settled into the region this morning...with very
comfortable temperatures observed with readings in the middle to upper 40s/near
50 as of 09z. These cool temperatures will be short lived however as the surface
ridge departs the area to the east today...resulting in a south-southeast low level
flow in its wake. Infrared satellite imagery is depicting an extensive layer
of ac over lower East Texas/west-central la...with additional elevated moisture
associated with a weak shortwave now nosing into the Texas/OK panhandles
expected to traverse east along the Red River valley today. Mosaic radar
imagery indicates some scattered elevated returns over central/southern OK this
morning...with mainly trace amounts recorded per surface observation. As this
shortwave shifts farther east into southeast OK/SW Arkansas...it will encounter a much
drier low level air mass...but still can/T rule out very light quantitative precipitation forecast
amounts over NE Texas/southeast OK/SW Arkansas. In addition...steep lapse rates aloft
may yield isolated thunder...thus have retained mention of slight
chance probability of precipitation for these areas through the afternoon. This shortwave
should shift east of the region by this evening...but should see a quick
return of 850mb moisture NE into East Texas/southeast OK this evening...before
spreading east-northeast into extreme northern la/SW Arkansas overnight. Min temperatures tonight
will be noticeably milder than the last couple of mornings...owing to
the increasing warm advection and cloud cover. Moisture advection will
actually deepen with time during Wednesday...as a warm front begins to
lift north across the region.

Have discounted the 00z NAM as it is an outlier /and much more
aggressive than earlier runs/ in generating convection along/north of the
warm front Wednesday...with the convection actually reinforcing the
front back S during the day. Instead...have gone with a blend of the
GFS/European model (ecmwf) with scattered convection developing by afternoon over central/southern
OK near a convergence bullseye ahead of a deepening surface low along the
dryline over West Texas...and a weak cold front that will drift S and
temporarily stall from north central OK into northern Arkansas. By afternoon...the warm
front should extend southeast along the Red River on North Texas into NE Texas to near
the Arkansas/la line. The placement of the front will be important as it
will focus additional convergence during the afternoon and
evening...with large scale forcing increasing later in the day ahead
of a shortwave that will traverse OK/the Red River valley into Arkansas
overnight. Moderate instability will develop Wednesday afternoon
across the warm sector...with MLCAPES of 1500-2000 j/kg and steep
elevated lapse rates in the 700-500mb layer. Backed surface winds ahead of
the surface low/dryline across north central Texas will contribute to updraft
rotation...with scattered convection possibly evolving into a mesoscale convective system as it
rides the warm front east-southeast into the region Wednesday afternoon and
especially Wednesday night. Have increased probability of precipitation to high chance for southeast
OK/western sections of East Texas Wednesday afternoon...tapering probability of precipitation to low
chance farther southeast over north central la where convergence is a bit weaker.
Should a mesoscale convective system develop over the Red River valley Wednesday
afternoon...this complex could accelerate southeast along the front posing a
damaging wind threat across the region as it taps the inflow from the
unstable air mass to the south-southeast before weakening late. Did increase probability of precipitation
to likely Wednesday night for much of the region...before this complex
of storms departs the region late.

The aforementioned weak cold front remains prognosticated to backdoor SW into
SW Arkansas/southeast OK Thursday morning before pulling up stationary over northern
la/East Texas by afternoon. This bndy and possible mesoscale boundaries from the
Wednesday night convection should again focus scattered convection Thursday
afternoon once instability is maximized. Scattered severe convection can/T be
ruled out Thursday afternoon/evening near/S of the front in the
greater instability...and strong deep layer shear in place. The
GFS/European model (ecmwf) are in agreement in shifting the stationary boundary back north as
a warm front Friday...with the warm sector expanding north into eastern OK/Arkansas
by afternoon. Meanwhile...the closed low in vc of the Southern California coast
should begin to eject NE into The Four Corners region and open up as
it enters the southern rockies and OK/Texas Panhandle region Friday afternoon.
An 80kt southwesterly middle level jet ahead of the ejecting shortwave should aid
in mixing the dryline quickly east into eastern OK to near the I-35 corridor
of central Texas...with increasing large scale forcing resulting in at
least scattered convective development across the region /and much of the
broad warm sector/ during the day. Have maintained high chance probability of precipitation
areawide Friday...with the convection expected to diminish from west to east
Friday night as middle level dry air quickly advects east into the region
beneath the ejecting upper trough. Could again see more scattered severe
convection Friday/Friday evening given the impressive
shear/thermodynamics...with the remnants of the dryline/surface trough
expected to quickly mix east across the region Saturday behind wrly surface
winds.

Should see maximum temperatures warm well into the 80s this weekend over much of
the region...with the medium range forecasts suggesting another
progressive southern stream upper trough traversing the Southern Plains late
Sunday and early next week. Have maintained slight chance probability of precipitation for now
as the forecasts are different in regards to the amplitude of the
trough.

Prelims to follow below...

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&&

Preliminary point temps/pops...
shv 75 58 80 64 / 20 10 40 60
mlu 75 55 80 63 / 20 10 30 60
deq 74 52 76 60 / 20 10 40 60
txk 75 57 78 61 / 20 10 40 60
eld 75 53 78 61 / 20 10 40 60
tyr 76 61 79 65 / 20 10 50 50
ggg 76 60 79 64 / 20 10 40 50
lfk 80 62 83 67 / 20 10 40 50

&&

Shv watches/warnings/advisories...
Arkansas...none.
La...none.
OK...none.
Texas...none.
&&

$$

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