Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion...corrected preliminary point probability of precipitation 
National Weather Service Mobile Alabama
1216 PM CST sun Dec 21 2014

Update...late morning satellite imagery and surface observations
continue to show an extensive deck of low level cloud cover remaining
in place across the County warning forecast area...with additional considerable middle/high
level cloudiness streaming overhead. These clouds have limited
insolation across the area...especially over western and southern
portions where cloud decks have been lowest. We have updated to
lower high temperatures this afternoon across some locations.
Otherwise...the stream of deeper moisture remains focused offshore
late this morning...with radar showing little in the way of echoes
across inland portions of the forecast area. Still cannot rule out an
isolated shower near the coast or interior eastern part of the County warning forecast area
this afternoon...but probability of precipitation have been adjusted downward. Minor
adjustments were otherwise made to inland winds...temps...and
dewpoints to account for the latest trends. /21


21.18z terminal forecast discussion...
IFR to MVFR ceilings and occasional reductions in visibility to MVFR
levels should continue near the coast...while MVFR ceilings should be
the rule further inland. Ceilings should deteriorate to IFR to LIFR
levels late this evening through early Monday morning...with
additional potential for MVFR to IFR visibility reductions. Isolated
to scattered rain showers may develop near the coast late tonight and
early Monday morning before developing inland during the day Monday.
Northeast to east winds around 5 knots should otherwise prevail. /21


Previous discussion... /issued 634 am CST sun Dec 21 2014/

Marine...updated current marine forecast to allow the Small Craft
Advisory for the open Gulf waters to expire. Small craft should
exercise caution over the open Gulf waters this morning. A light
southerly flow develops by Monday then builds ahead of a strong cold
front Tuesday and early Tuesday night. A strong west to northwest
flow develops in the wake of the front Wednesday through Thursday
morning. A Small Craft Advisory will likely be required for the
southerly flow ahead of the front on Tuesday...and in the wake of
the front Wednesday through early Thursday. 32/ee

Previous discussion... /issued 447 am CST sun Dec 21 2014/

Short term (today through Tuesday night)...for today and
tonight...a broad middle to upper trough over much of the central Continental U.S.
Will shift eastward through tonight leading to better lift and
forcing in the middle levels on the east side of the trough generally over
the adjacent Gulf waters of Alabama and nwfl later today and tonight
shifting slowly northward through Monday morning. At the surface latest
buoy observation and satellite loops continue to show a weak low generally
located south of nwfl stalled along a weak frontal boundary
stretching from east to west over the northern Gulf generally well
offshore. This system eventually fills and moves northward as the
upper trough continues to shift east through early next week. For today
skies remain mostly cloudy today and tonight with increased rain
chances initially starting over the adjacent Gulf waters later today
shifting northward affecting mostly the eastern half of the County warning forecast area late
today and tonight. For now will continue to mention this precipitation as
light to moderate rain generated mostly from weak layer lift on the
favorable side of the upper system approaching from the west. As for
temperatures will continue to lean towards the cooler MOS values today
undercutting these values 2 to 3 degrees lower than the current mav
guidance due to clouds and persistence. For tonight lows will be on
the warmer side of the current MOS numbers going 1 to 2 degrees
warmer for most areas... especially to the east for better
consistency with surrounding weather forecast offices. Highs today will be warmer then
last couple of days ranging from the upper 50s to lower 60s for most
areas followed by low temperatures in the middle to upper 40s for most inland
areas and the lower 50s along the immediate coast. 32/ee

For Monday through Tuesday night...a longwave trough over the central
states amplifies significantly through Tuesday then begins to
advance eastward somewhat Tuesday night. An associated surface low
over the northern plains Monday morning occludes with the upper
system on Tuesday while heading towards the Great Lakes
region...meanwhile bringing a cold front across the lower
Mississippi River valley. A surface low will have developed on the
frontal boundary on Monday over Texas but there is now considerable
uncertainty on the path of this feature through Tuesday night. The
GFS and European model (ecmwf) move the surface low down to extreme southern Texas
Monday night then move the system northeast to near southeast
Louisiana by Tuesday evening...then continuing northeast to northern
Alabama late Tuesday night and bringing the cold front through the
forecast area in the process. The NAM and Gem on the other hand are
closer to previous solution of the European model (ecmwf) and take the surface low to
the lower Mississippi River valley near midday Tuesday then to near
the Ohio River valley Tuesday night.

Despite uncertainty with the path of the surface low...a warm front
still advances northward through the forecast area ahead of the
approaching cold front bringing surface dewpoints in the lower 60s
into much of the area by late Monday night. The concern is that the
southerly trajectory of the surface low in the European model (ecmwf)/GFS may lead to
a secondary warm front developing over the northern Gulf...and while
high probability of precipitation will be in store for the area either way Tuesday into
Tuesday night...this secondary warm front would be the focus for the
best shear and instability thus limiting the severe/strong storm
potential over the forecast area.

With this not expecting a secondary warm front to
develop over the northern Gulf thus maintaining the potential for
strong to possibly severe storm development in the forecast area.
0-3 km MUCAPES of 500 j/kg will develop over much of the western and
coastal portion of the area by Monday night...and as high as near
1000 j/kg near the immediate coast...with values of 500-1000 j/kg
expected for Tuesday. 0-1 km helicity values of 100-150 m2/s2 will
be present on Monday increasing to 150-250 m2/s2 by Tuesday as the
850 mb jet increases from 25 knots to 30-40 knots. With this
environment...may see strong storm development as early as late
Monday night over the western and coastal portions of the area as
deep layer lift increases...then with a line of convection expected
to move across the forecast area during the day on Tuesday. As
there is uncertainty with the evolution and timing with this going to leave the timing wording as is in the hazardous
weather outlook which has some severe storm potential mainly Tuesday
afternoon and evening...and let the day shift reassess. Will have
slight chance to chance probability of precipitation on Monday increasing to good chance
across the area Monday night...then categorical probability of precipitation follow for
Tuesday then taper off through Tuesday night with the passage of the
cold front. Mild temperatures are expected with highs warming to
the lower 70s by Tuesday. Warm overnight lows Monday night ranging
from the upper 50s well inland to lower 60s near the coast will be
about 20-23 degrees above normal. Cooler temperatures follow for
Tuesday night but still well above normal...ranging from the lower
50s well inland to middle 50s near the coast. /29

Long term (wednesday through saturday)...probability of precipitation continue to taper off
on Wednesday in the wake of the front with dry conditions for
Wednesday night through Thursday night. Another cold front
meanwhile advances across the plains and is expected to move across
the forecast area on Saturday. Will have small probability of precipitation returning to
the area on Friday and increasing to chance probability of precipitation by Saturday. Highs
will be in the middle 50s inland to near 60 closer to the coast
Wednesday and the middle 60s for Friday...then trending
somewhat cooler on Saturday with highs in the lower to middle 60s.
Lows Wednesday night will be in the middle to upper 30s then trending
warmer to the upper 40s inland to lower 50s closer to the coast by
Friday night. /29

Marine...a broad area of low pressure will continue to drift across
the north central Gulf and weaken through tonight. A moderate to
occasionally strong northeast wind will diminish today and early
tonight as the surface low gradually weakens. Southerly winds and
seas will develop on Monday and build through early Tuesday night
ahead of a strong cold front approaching from the northwest.
Scattered to numerous showers and thunderstorms are expected ahead
of the front Tuesday and Tuesday night with some storms becoming
strong to possibly severe. The cold front is expected to move across
the marine area by early Wednesday leading to a strong west to
northwest flow over the marine area early Wednesday through midday
Thursday. Small craft advisories will be likely ahead and in the
wake of the cold front by midweek. 32/ee

12z issuance...LIFR to IFR ceilings through about 20.14z followed by IFR
to MVFR ceilings through 21.06z then LIFR to IFR ceilings and visibilities
through 21.12z. Light rain mostly well offshore early this morning
will shift northward over the central Gulf region especially along
the immediate coast late today and tonight. Winds will continue from
the northeast at 5 to 10 knots today shifting mostly east at 4 to 8
knots through 21.12z. 32/ee


Preliminary point temps/pops...
Mobile 58 51 70 62 71 / 10 30 30 50 80
Pensacola 57 53 69 62 71 / 20 50 40 50 90
Destin 59 54 68 63 71 / 30 60 40 50 90
Evergreen 62 48 67 60 71 / 10 50 30 50 80
Waynesboro 57 45 67 59 70 / 10 10 20 40 80
Camden 61 47 67 58 71 / 10 20 30 40 80
Crestview 60 49 68 59 70 / 20 50 40 50 90


Mob watches/warnings/advisories...



National Weather Service Glossary of Abbreviations