Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Mobile Alabama
1222 PM CDT Wednesday Jul 29 2015
Updated for latest aviation discussion below. /02/
A southward-moving disturbance will combine with ample instability
and plenty of low-level moisture to result in the development of
rain showers and thunderstorms and rain across the region this afternoon. As such...expect
MVFR flight categories (due to lower cigs) to likely develop across
parts of the region this afternoon...mainly in vicinity of any
convection. The greatest odds of lower flight categories will likely
be across S Alabama and northwest Florida.
Gusty winds associated with the stronger thunderstorms and rain will likely be the
main impact to aviation interests. Convective coverage will likely
diminish after 30/0000 UTC with the loss of daytime heating...with
additional rain showers/thunderstorms and rain possible tomorrow.
Outside of any convective activity...look for VFR flight categories
and west-northwest winds 5-10 kts. /02/
Previous discussion... /issued 1114 am CDT Wednesday Jul 29 2015/
latest water vapor imagery...and subjective analysis...indicates a
shortwave trough rotating southward across eastern Alabama/western
Georgia. Upstream sounding analysis indicates a pretty healthy slug
of boundary layer moisture is associated with this feature...with
observed precipitable water values hovering around the 2 inch mark
(plus or minus 0.10 inch).
Expect this feature to initiate convection as it enters the
relatively uncapped atmosphere across our south-central Alabama
counties early this afternoon. MLCAPE values already exceed 2000
j/kg in many locations at mid-morning...and will likely increase
heading into the afternoon. In addition...mid-tropospheric lapse
rates should steepen this afternoon as a pocket of cooler
temperatures associated with the shortwave trough enters the region.
The sea breeze is still expected to move onshore. This will result
in convective development closer to the coast by mid-afternoon.
Modified soundings for this afternoon indicate MLCAPE values around
2500 j/kg...precipitable water values remaining around 2 inches...
and freezing levels between 15500 and 16000 feet. Putting all of this
together...we cannot rule out the development of isolated severe
thunderstorms...especially along and east of the ih-65 corridor.
Adjustments to the timing and evolution of precipitation
probability/weather grids have been made through tonight. Greater
odds have introduced between 1 and 4 PM across parts of south-
central Alabama...then blended into (more-or-less) the pre-existing
precipitation probability grids. The enhanced odds of seeing
convection have been extended until 8 PM...with no change at this
Otherwise...the forecast appears to be on track. Isc grids have
been sent and published to the internet. Updated text products will
Depending on where you're located...there is a moderate to high risk
of wet microbursts this afternoon. /02/
Previous discussion... /issued 400 am CDT Wednesday Jul 29 2015/
Short term [today through Thursday night]...for the first two periods of
the forecast...biggest item to deal with is a shortwave moving south
across the tenn River Valley and southeastern Continental U.S. Today into
tonight. The high res end of the guidance (hrrr/nmm/arw, etc) are
advertising an mesoscale convective system setting up over northern Alabama today...then moving
south towards the Gulf of mex into tonight. The medium range guidance
is advertising the usual rain showers/thunderstorms and rain firing off with daytime heating.
Either way one cuts it...scattered to numerous thunderstorms and rain expected today
over most of the forecast area...lasting into the overnight hours.
For temperatures...with upper level high pressure in firm control over the
Southern Plains and lower Mississippi River valley...temperatures above
seasonal continue. Combined with a very humid airmass (precip h20
values > 2")...heat indices rising to around advisory criteria today
is expected...so continuing the previous issued heat advisory.
On to any severe weather...with a very hot/moist/unstable airmass (capes
topping 3500j/kg) over the area today...anticipating some of the
thunderstorms becoming strong to severe...with damaging winds the
expected threat. This will be especially true with the mesoscale convective system expected
to move south across the forecast area.
There is a high risk of damaging downbursts across the area today
into tonight. /16
Thursday and Thursday night...amplified middle level ridge axis persists
from Texas...eastward into the southeast...while broad high level
trough advances eastward across the middle Mississippi River valley and
Great Lakes. At the surface...a weak frontal zone becomes quasi-
stationary from the middle Atlantic to southeast Texas. With the
presence of this boundary draped across the central Gulf Coast being a
focus for ascent...coupled with high deep layer moisture and daily
atmospheric instability...results in a chance of showers and storms
on Thursday. Showers and storm chances carry over into the evening.
Given precipitable water values of 2... as high 2.3 inches being
well above climatology for the end of July (1 to 2 Standard
deviations above the mean)...the stronger storms will be quite
efficient in dropping some locally heavy rains. Other hazards in and
near storms will be brief strong wind gusts and frequent lightning.
Daytime highs on Thursday look to be much more suppressed over the
southeastern zones ranging from the upper 80s to lower 90s.
Elsewhere...middle 90s in the forecast. Little change in overnight lows
Thursday night...middle to upper 70s. /10
Long term [friday through tuesday]...heading into the weekend...the
global models are in good agreement on showing the erosion of the
eastern flank of the upper ridge as trough digs southward across the
Appalachians. The weak surface front/trough remains nearly stationary
through much of the weekend...with maybe a slight southward advance
by Sunday. Chances of showers and storms continue area-wide on
Friday...but will trend southeast Saturday and Sunday...with the
higher chances becoming oriented along and southeast of I-65.
For Monday and Tuesday...with surface trough out over the Gulf and a
reduction in deep layer moisture...chances of showers and storms stand
at 20% or less.
Daytime highs this weekend generally in the lower half of the 90s
over the interior to middle/upper 80s at the beaches. Some upward
moderation by 1 to 3 degrees in these numbers is forecast Monday and
Tuesday. For overnight lows...mostly in the middle 70s interior to upper
70s/near 80 at the beaches. /10
12z issuance...general VFR through the forecast. Exceptions being
MVFR to IFR ceilings/visibilities in scattered to numerous rain showers/thunderstorms and rain expected
to develop over northern Alabama today and move south across the forecast area
this afternoon and evening. /16
Marine...a surface ridge stretching west over the Gulf of mex will
create a general westerly flow over area coastal waters through the
week. A surface boundary moving south across the southeastern states
Thursday will move to near the coast by Thursday night...tightening
the pressure gradient over open waters...bringing a more moderate
westerly flow Thursday night into the weekend. /16
Preliminary point temps/pops...
Mobile 96 75 92 77 91 / 60 60 40 30 50
Pensacola 96 77 91 78 89 / 60 60 40 30 40
Destin 93 79 89 79 87 / 50 50 50 30 40
Evergreen 96 74 94 75 93 / 60 60 30 30 40
Waynesboro 97 74 96 75 93 / 40 40 30 30 40
Camden 95 74 96 76 92 / 50 50 30 30 30
Crestview 97 75 92 75 91 / 60 60 40 30 30
Alabama...heat advisory until 7 PM CDT this evening for Baldwin central-
Baldwin coastal-Baldwin inland-Butler-Choctaw-Clarke-
Florida...heat advisory until 7 PM CDT this evening for Escambia coastal-
Escambia inland-Okaloosa coastal-Okaloosa inland-Santa Rosa
coastal-Santa Rosa inland.
MS...heat advisory until 7 PM CDT this evening for George-Greene-