Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Mobile Alabama
343 PM CDT Wednesday Oct 1 2014

Short term (tonight through Saturday night)...for tonight...surface
high along the Atlantic coast will continue to push into our area
from the northeast through tonight. Model calculations indicate upper
ridging will increase slightly through tonight meaning little to no
chance of rain here through sunrise Thursday morning. As we move into
the day Thursday...the western edge of the surface ridge will have
already brought a deep return flow with isolated showers through most
of the first 24 hours of the short term. Skies partly to mostly
cloudy through tonight...with patchy fog possible overnight.

On Thursday afternoon...GFS physical output indicates a small
anomalous feature south of Orange Beach bringing a rise in
precipitable water 2.1 to 2.2 inches and increasingly negative
lifted index of -5 albeit with a modest cape around 1000 to 1500.
With the modest cape there...other signals...such as positive
q-vector divergence and downward Omega...are convincing enough to
counter these two items...especially considering the size and short
lived nature of the feature. The MOS guidance is not picking it up
for probability of precipitation...nor are we convinced it warrants increased probability of precipitation.

Generally light east to southeast wind today and tonight. For the
rest of the period we looked at the MOS winds calculated for the
10.12z run and found them to be not significantly different from
what the previous shift we left the wind as they had it
after Friday night.

Lows and highs respectively tonight and tomorrow will be just about
seasonable for October...which on average are around the middle to
upper 70s for highs and middle 50s for lows. 77/blowing dust

For Thursday night through Saturday night...a longwave trough over the
central states continues to strengthen while advancing into the
eastern states as an upper low develops over the northern plains
Thursday night then deepens significantly while moving to north of
the Ohio River valley Friday night into Saturday morning. Of
interest is that the upper low becomes particularly intense by
Friday night...deepening to 4 Standard deviations below
climatological normals Friday night into Saturday. The upper low
ejects off to the northeast Saturday night while the longwave trough
amplifies somewhat more over the eastern states. A surface low will
have developed over the Central Plains in response to this system
and deepens while moving to the Great Lakes region Thursday night
and well to the north on Friday...meanwhile bringing a strong cold
front that moves through the forecast area Friday afternoon into
Friday evening. A dome of cold high pressure builds into the region
in the wake of the front...settling over the forecast area Saturday
night and brings the coldest temperatures so far this season.

Despite the intense upper low and overall strength of the upper
trough...with the surface low ejecting so far to the north...the
southwesterly 850 mb flow over the forecast area maximizes around 25
knots late Thursday night into early Friday morning then decreases to
around 20 knots as the the front approaches and then moves through.
This results in 0-1 km helicity values reaching at best 70-100 m2/s2
late Thursday night into Friday morning then decreasing quickly to
50 m2/s2 or less until the front moves through. The best
instability present will be near 2000 j/kg near the immediate coast
tapering to values near 1100 j/kg well inland...and with a very
moist environment in place ahead of the front with precipitable
water values around 1.8-2.1 expecting general
thunderstorms with some strong storms capable of strong gusty winds
and heavy rainfall. The current hazardous weather outlook mentions
a possible isolated strong to severe storm on Friday which looks OK
at this point.

Will have chance probability of precipitation for most of the area Thursday night then likely
probability of precipitation continue for Friday as the front moves through. Small probability of precipitation
follow generally east of I-65 Friday evening with otherwise dry
conditions through Saturday night.

Lows Thursday night will be rather mild for this time of year and in
the lower to middle seventies...which is at least ten degrees above
normal. Highs on Friday will be near normal and in the lower to middle
eighties then lows Friday night in the wake of the cold front will
be near normal values ranging from the lower to middle sixties near the
immediate coast to lower to middle fifties well inland. Saturday will
be a few to several degrees below normal with highs in the middle to
upper seventies. The coldest temperatures so far this season are in
store for the area Saturday night as the surface high settles over
the area with light winds maximizing radiative cooling and resulting
in lows ranging from the lower to middle forties over inland areas
ranging to around fifty near the immediate coast...which will be
about 13-15 degrees below normal. These unseasonably cold
temperatures will be near the record values for Mobile and Pensacola
but expected to be about two degrees above. Here is a comparison of
the forecast and current records for Mobile and Pensacola Sunday
morning (saturday night):

Forecast record
Mobile 46 44 (2010)
Pensacola 51 49 (1987)


Long term (sunday through wednesday)...the longwave trough persists
over the eastern states at least through Monday night then ejects
off to the east with broad upper ridging building into the region
through Wednesday. A surface high over the area Sunday morning
moves off into the Atlantic allowing for a return of southerly winds
and a return to seasonable temperatures by Monday and slightly
warmer temperatures by Wednesday. Dry conditions are expected
through the period except have kept small probability of precipitation mainly for Monday
night and Tuesday as a series of shortwaves move across the area. /29


for 01/18z and 02/00z taf cycles...VFR conditions continue
this afternoon and evening. Starting with broken thick cirrus near
Florida 250 with scattered to broken lower ceilings...but generally above
Florida 030. Some areas of IFR conditions possible late tonight with fog
or mist down to 3 miles...possible low stratus to Florida 015...we
believe it likely enough to warrant a prevailing group. Generally
light east to southeast surface winds though tonight and Thursday
morning...but the clouds and convection will be increasing Thursday
afternoon west of Interstate 65. 77/blowing dust


Marine...a surface high northeast of the coastal waters will drift
east off the US Atlantic coast today through early Friday. This
feature will bring light to moderate winds over the region through
early Friday. Meanwhile a cold front will approach from the west and
move across the coastal waters late Friday afternoon and evening. A
strong high will build behind the front bringing moderate to strong
offshore winds and building seas as the front moves offshore.
Numerous showers and thunderstorms expected Thursday and Friday as
the front approaches and passes across the marine area. The wna did
not quite catch the effects of enhanced Post frontal wave heights in
which the northeast propagating group and the southward propagating
group intersect. Nor were the effects of downward momentum transport
under cold advection very prominent. However...the Swan model
handled the process beautifully with a small maximum wave height
area calculated just behind it. We smoothed the boundaries and
restored what we saw the Swan trying to do there. 77/blowing dust


Preliminary point temps/pops...
Mobile 71 87 73 85 58 / 20 40 50 60 20
Pensacola 72 85 74 84 62 / 10 30 40 60 30
Destin 77 83 74 84 64 / 10 20 30 60 30
Evergreen 67 90 71 83 53 / 10 30 40 70 20
Waynesboro 65 89 71 83 51 / 20 50 60 60 10
Camden 66 90 72 83 51 / 10 30 50 70 10
Crestview 68 88 73 85 57 / 10 20 30 70 30


Mob watches/warnings/advisories...


National Weather Service Glossary of Abbreviations