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Area forecast discussion 
National Weather Service Birmingham Alabama
954 PM CST sun Mar 1 2015

Update...
evening update.

&&

Discussion...

An mesoscale convective vortex moved across South Alabama this evening and this is where
most of the rain has fallen since sunset. The impulse is weakening
and the rain shield is becoming smaller and less defined. Some
light rain showers were observed across north Alabama. The rain
should become more focused across extreme north Alabama after
midnight as a cold front approaches the area. Surface
temperatures will remain steady or possibly fall a few more
degrees overnight...due to cloud cover and strong inversion just
above the surface.

58/rose

&&

Aviation...
00z taf discussion.

Area of heavier rain and a few storms moving east across SW central
Alabama south of tcl. Some light to moderate rain is possible at
any terminal this evening with the better chances in the north
overnight. Ceilings are expected to fall back to IFR area wide this
evening and then likely LIFR in most locations late tonight. With
continued isentropic lift and increased low level moisture not
seeing any signs of major improvement on Monday. Therefore will
keep prevailing IFR through the day Sunday. Winds will generally be
light and variable through the period as a frontal boundary moves
into and stalls across the forecast area.

19



&&

Previous discussion...
/issued 600 PM CST sun Mar 1 2015/

Current water vapor imagery and RUC upper air analysis indicates
rather zonal flow across the southeastern US with a cut off low
across Southern California with another trough moving across the
northern plains into the Great Lakes. At the surface...a cold front
is located just back to the west in Mississippi and is the focus for
showers and convection to the west. Looking at the radar this
afternoon there are a few thunderstorms in Mississippi where the RUC
analysis indicates very low amounts of most unstable cape. This weak
instability decreases to almost nothing into southwestern portions
of central Alabama. However...the thunderstorm activity has held
together fairly well so far and have mentioned a slight chance for
thunderstorms this afternoon and evening across far southwestern
portions of central Alabama. The surface front will begin to move to
the southeast later tonight. Due to this...rain chances will
increase across the northwestern half off the area. Precipitable water values will
also rise tonight to around 1.25-1.50 inches. These numbers are
around the 90th percentile for early March and went ahead and
increased probability of precipitation during the overnight period. Further to the
south...model soundings indicate that low level moisture will be
plentiful and fog formation will be possible in these areas after
midnight. With widespread cloud cover and the front located just to
the northwest of the area throughout most of the night...expect
overnight lows will be mild and in the upper 40s to low 50s.

The front will bisect the forecast area for much of the day on
Monday with a decent spread in temperatures across central Alabama.
Expect temperatures will range from middle 40s across the northwestern
counties to middle to upper 60s across southeastern portions of the
forecast area. This front will continue to provide a focus for
showers to develop along during the day tomorrow. Precipitable water values will
remain around 1.5 inches and have increased probability of precipitation for Monday.

Monday night into Tuesday the closed low across Southern California
begins to eject to the east as an upper level trough dives across
southern Canada into the northern plains. At the surface...the high
pressure moves to the northeast with a Lee low developing. Surface
winds begin to shift to more of a southeasterly direction and will
push the front northward as a warm front across the forecast area.
Could see some fog yet again right along this front Monday night
into Tuesday morning with saturated low levels. Temperatures should
warm during the overnight hours as the front lifts northward with
mild overnight lows once again in the upper 40s to middle 50s. Tuesday
will be a warm day with upper level heights continuing to increase
as the troughs further to the west become close to phasing. Strong
southerly winds will advect in warm and moist air with
temperatures climbing into the 70s areawide. Lift will be somewhat
limited with the warm front moving well to the north and the cold
front still well to the west. Therefore...have went with the lower
probability of precipitation on Tuesday.

Tuesday and beyond is where the forecast get very interesting once
again expect central Alabama will go from temperatures in the upper
70s and low 80s with a few thunderstorms Tuesday and Wednesday
afternoons to a potential winter storm late Wednesday night into
Thursday morning. The main difference in the model solutions for the
middle-week system is timing. Right now the GFS is a good 6 hours faster
than the rest of the models. The Gem and European model (ecmwf) are much slower with
the front. In addition...the operational GFS appears to be one of if
not the quickest of all the GFS ensemble members. The slower model
solution would be a bit more favorable for the potential for strong
to possibly severe storms. The slower models allow more time for
instability and lapse rates to steepen during the day on Wednesday
ahead of the front. In addition...low level winds remain more out of
the south in the slower models which will increase the low level
shear ahead of the front. For now...have not mentioned any severe
weather in the severe weather potential statement but will continue to monitor the model trends.

As for the chances for winter weather...have slowed down the arrival
of wintry precipitation until after 06z Thursday across areas
generally along and north of the I-59 corridor. Regardless of
timing...the synoptic setup is rather similar. The phasing of the
upper level troughs just to the west across the Southern Plains
creates an incredible upper level jet of over 180kts across the Ohio
River valley into the northeast. This situation continues to put
central Alabama in the right entrance region of the strong jet after
the front moves through the area. This will provide synoptic lift
across the forecast area as cold air at the surface moves in behind
the Arctic front. All of the model soundings have a warm nose
sufficient for complete melting with freezing rain being the primary
precipitation type as temperatures fall below freezing. There could be
enough moisture left over towards the end for a sleet/snow mixture
but expect the majority of the winter precipitation will be in the
form of freezing rain. The GFS is the most aggressive by far with
the possibility of freezing rain all the way down to the I-85
corridor. The Gem and European model (ecmwf) are a bit less aggressive with freezing
rain down to or slightly south of the I-59 corridor. Looking at the
incredible jet believe the quantitative precipitation forecast amounts from the European model (ecmwf) may be a bit
underdone. Even based of the slower solution...significant ice
accumulations would still be possible along and north of the I-59
corridor. Again...strong winds of 10-20 miles per hour will advect in the
Arctic air will further exacerbate any icing issues on trees and
powerlines. Please stay tuned to the latest forecast with the event
still 3-4 days out.

All the models show very impressive cold air advection behind the
system with a few locations north of I-20 possibility staying below
freezing all day Thursday. If there is snow or ice on the ground
Friday morning expect temperatures in the single digits will be
possible north of the I-20 corridor. Expect some locations may not
make it above freezing until Friday afternoon as temperatures begin
to moderate late in the week and into the weekend. Have continued
low probability of precipitation over the weekend with the European model (ecmwf) showing a Gulf low
developing. The GFS shows the low suppressed for now and will
continue with low probability of precipitation mentioned.

05/Massachusetts

&&

Preliminary point temps/pops...
Gadsden 47 52 47 68 61 / 90 70 60 40 40
Anniston 48 55 51 69 62 / 70 60 60 30 40
Birmingham 51 53 51 71 63 / 90 70 60 30 40
Tuscaloosa 52 55 52 74 64 / 90 70 50 30 40
Calera 51 57 54 72 63 / 90 60 40 30 40
Auburn 48 64 57 72 62 / 40 30 40 30 20
Montgomery 52 65 58 75 64 / 60 30 30 20 20
Troy 51 65 58 75 63 / 40 20 20 20 20

&&

Bmx watches/warnings/advisories...
none.

&&

$$

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