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Area forecast discussion 
National Weather Service Huntsville Alabama
559 am CST Sat Feb 28 2015

Update...
for 12z tafs

&&

Discussion...
/issued 554 am CST Sat Feb 28 2015/
a seasonably strong middle-level southwest flow regime prevails across
the forecast area this morning...as the region remains between a
positively-tilted longwave trough extending from the Pacific coast
into eastern Canada and the Atlantic subtropical ridge centered north
of Puerto Rico. Very little change in this pattern is expected
through Sunday...with several lower amplitude disturbances prognosticated to
track east-northeastward from the Southern Plains into the Ohio
Valley expected to have little influence on local weather conditions.
Deterministic guidance from the GFS/ECMWF/NAM is in strong agreement
that 500-mb heights will gradually rise through the day on Sunday...
as the subtropical ridge retrogrades across the Greater Antilles and
eventually becomes centered across the Yucatan Peninsula tomorrow.

At the surface...a ridge -- currently centered across eastern
Ohio/western Pennsylvania -- is forecast to shift slowly eastward
today. Surface pressure rises will become consolidated in the Lee of
the southern Appalachians as this occurs...which will induce a
southeasterly return flow and warmer afternoon temperatures across
northern Alabama/southern middle Tennessee. Although the warming
trend will be slow this morning given overcast altostratus deck in
place...temperatures should warm into the lower 50s as this cloud cover
dissipates during the afternoon. A persistent south-southwesterly
low-level jet around 20-25 knots will likely generate sufficient lift
to result in the formation of a solid deck of lower stratus clouds by
late this evening -- with patchy drizzle possible region-wide Sunday
morning. Chances for light rain will slowly increase for all areas by
Sunday afternoon...although with some model guidance indicating the
presence of a dry layer between 3-7 kft will only increase probability of precipitation into
the chance category.

Meanwhile...global models suggest that a strong northern stream
trough will dig southeastward into the northern plains on Saturday
night...and across the upper Mississippi Valley and Great Lakes on
Sunday/Sunday night. As this occurs...a deepening surface low will
lift northeastward through New England...dragging a strong cold
front through the region late Sunday night/early Monday morning. A
narrow band of light showers will likely accompany frontal
passage...with a fairly widespread postfrontal light rainfall event
expected as 20-25 knot westerly flow north of the subtropical ridge
and atop the cold airmass will lead to efficient low/mid-level
ascent. This regime will continue on Monday...and with thick clouds
and moderate northeast winds temperatures will only warm into the
middle 40s.

In the extended period...models are in good agreement in forecasting
a southern stream trough to eject rapidly northeastward from the
southwestern deserts into the central rockies on Monday night --
inducing low-level cyclogenesis across eastern Colorado/western Kansas. A warm
front will surge northward over the region...and we have included low
chance probability of precipitation. There is some concern for the development of elevated
convection -- mainly between 06-12z Tuesday -- as forecast soundings
all suggest the development of fairly steep lapse rates in the
700-500 mb layer. The region will be well into the cyclones warm
sector on Tuesday...with gusty south winds boosting temperatures into
the middle/upper 60s despite abundant cloud cover. Scattered
showers/storms will be possible...but coverage should be on the low
side with only weak deep forcing for ascent. Intense low-level
warm/moist advection on Tuesday night will likely support a rapid
increase in the coverage of convection...with this expected to
continue on Wednesday. Although sb instability parameters will be
generally weak...dewpoints will rise into the 60-65 range beneath a
50-60 knot southwesterly low-level jet -- raising concern for
strong/severe storms.

A powerful Arctic cold frton will begin to push southeastward into
the area late Wednesday afternoon...and continue slowly
southeastward exiting the southern counties just before midnight.
However...with axis of the broader longwave trough still to the
west...very strong deep-layer forcing for ascent will occur for at
least 12-24 hours in the wake of frontal passage -- supporting
widespread moderate/heavy postfrontal precipitation.
Unfortunately... this may spell disaster for portions of the
Tennessee Valley...as vertical profiles become highly supportive of
freezing rain late Wednesday night into Thursday morning -- possibly
transitioning briefly to sleet and eventually light snow around
sunrise Thursday. This event certainly has the potential to carry a
much greater impact than the record breaking snowfall that occurred
just a few days ago -- given the simultaneous occurrence of
significant ice accumulations and brisk north winds -- and will be
watched closely over the coming days.

70/dd

&&

Aviation...
for 12z tafs...

VFR conditions will prevail across the forecast area through at least
03z this evening. Middle level ceilings will persist through the
day...remaining above 9000 feet. Winds out of the northeast will become
more southeasterly as the day progresses. Low clouds will approach
the area late tonight as isentropic lift increases across the region.
2000 feet ceilings will overtake the terminals by 03z...and then lower
to 900 feet by 09z on Sunday.

&&

Hun watches/warnings/advisories...
Alabama...none.
Tennessee...none.
&&

$$

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at weather.Gov/Huntsville.

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