Scientific Forecaster Discussion
Area forecast discussion...update
National Weather Service Peachtree City Georgia
1102 am EST Thursday Mar 5 2015
Late morning update...
regional observation show the shallow cold air to the northwest being blocked by a
Ridgeline that runs from NE Alabama into east central Tennessee... with
temperatures west of the Ridgeline at or below freezing... and temperatures east
above freezing... with cha...dnn... and rmg still in the middle to
upper 30s. Still expect the cold air to eventually spill over the
Ridgeline and possibly give northwest Georgia a brief window of freezing temperatures
through early this afternoon. Also... before the rain field can push
east of northwest Georgia by early-middle afternoon... a mix of sleet/snow is still
possible. All this threat spreads into NE Georgia by early-middle afternoon
as the cold temperatures finally spread east before the rain field can push
east of NE Georgia. Although warm ground temperatures and help from daytime
heating will help hold any wintry precipitation accumulations down to maybe
a dusting of sleet/snow or a brief glaze of ice at best... will have
to play it safe and hold onto the advisory has posted for now.
Surface temperatures fall below freezing later this afternoon through the
evening for most of north Georgia...even south of Atlanta by early
evening... so the black ice threat begins early across north Georgia and
continues overnight. If we do clear the advisory area with the late
afternoon forecast package... we will continue with a Special
Weather Statement to cover this black ice threat this evening and
overnight. Stay tuned. /39
Previous discussion... /issued 351 am EST Thursday Mar 5 2015/
Short term /today through Friday/...
cold front currently stretches from the southern middle Atlantic to the
northwest corner of Georgia to the Louisiana Gulf Coast. The front has made
slow progress over the last few hours...but have seen a noticeable
increase in precipitation behind the boundary.
Currently...temperatures are warm enough to support all rain...but
that will change later this morning for portions of the County warning forecast area.
Models still in good agreement with the timing of the front. The NAM
brings in the colder air at the surface a few hours earlier than the
00z run last night...but otherwise...no major changes to the current
forecast. The cold air at the surface will begin to infiltrate the
northwest corner of the County warning forecast area around 12z this morning...and spread eastward
through the late morning and afternoon. The colder air aloft doesn't
look to come in across the north until late in the afternoon or
early evening...when the deepest moisture begins to push southward.
Model soundings still indicate the /predominate/ ptypes will be rain
transitioning to sleet and freezing rain. In addition...the models
are still showing some very shallow moisture lingering for a few
hours after 00z...so have added some frizzle (freezing drizzle) to
the forecast.The highest sleet accumulations are still clocking in
around one quarter of an inch...but most locales will be below this
value. Ice accumulations between one and two tenths are possible at
the higher elevations...while amounts from a trace to a couple of
hundredths is possible across the lower elevations of far north
Georgia. These amounts will be just enough to create the potential
for light icing on roadways...especially on overpasses and bridges.
In addition...much colder temperatures overnight and early Friday
morning will create the potential for any lingering moisture on
roadways to turn into patchy black ice. Will be extending the ww
advsy longer in time and further east.
Across the remainder of the forecast area...it will be a soggy day.
The potential for black ice overnight will even extend into the
metropolitan area overnight and early Friday...so early morning commuters
should exercise caution.
High temperatures will be reached early this morning across much of
north Georgia...with falling temperatures through the afternoon. Temperatures in
the late afternoon are expected to be in the middle 30s to lower 40s
roughly along and north of a line from Braselton to LaGrange...so
even if the kiddos don't need a jacket this morning at the bus
stop...it will be cold enough for one by the time school lets out.
Small chances for precipitation may linger in the far southeast County warning forecast area
overnight...but temperatures should remain well above freezing in this
area. Friday should be dry...but some twenty or more degrees cooler
than previous days. Skies should remain cloudy or mostly cloudy.
Long term /Friday night through Wednesday/...
extended forecast period remains a bit vague concerning best chances
for precipitation. Upper flow is weak and generally zonal to
slightly southwesterly downstream from a persistent but weak trough
over the southwestern U.S. By middle-week this feature is showing signs
of shifting east which may help to increase rain chances a bit
through the end of the forecast. In the meantime any weak waves
moving out of this western trough lift through the region...but
timing and strength of any enhanced lift would be too uncertain
right now. All-in-all this just continues my trends from previous
cycles with slight chance to low chance probability of precipitation broad-brushed across
the forecast area by Monday through the end of the forecast. At the
surface...weak high pressure slowly migrates across the region
through early next week. GFS builds in a wedge type pattern by early
to middle-week and that is supported by the persistent upper-level
confluence over the middle-Atlantic region and New England. Ridge
center is not especially strong...1030mb...and source region is not
expected to be excessively cold so only slight moderation of
temperatures is expected at this time and by Thursday the confluent
upper region shifts well off the East Coast allowing The Wedge to
break anyway. As the surface ridge moves off-shore late in the
period return flow off of the Gulf of Mexico may allow for some
increase in instability...but little if any surface or upper-level
focus for organized thunderstorms appears present at this time.
/issued at 632 am EST Thursday Mar 5 2015/
cold front currently moving through northwest Georgia this morning. The front
will continue to slip slowly south this morning and afternoon.
Widespread rain is associated with the boundary and should last
well into the afternoon. Observation are not showing any IFR visibilities with
the rain...but ceilings are dropping to IFR. Will keep the IFR ceilings
until the rain begins diminishing. Gusty winds expected to last
much of the day also...and switch around to the northwest by the late
afternoon. MVFR ceilings expected overnight.
//Atl confidence...12z update...
medium confidence all elements.
Preliminary point temps/pops...
Athens 64 31 48 30 / 100 30 0 0
Atlanta 58 29 47 31 / 100 10 0 0
Blairsville 48 24 43 26 / 100 20 0 0
Cartersville 46 25 46 28 / 100 10 0 0
Columbus 66 33 53 33 / 100 20 5 0
Gainesville 57 29 46 30 / 100 20 0 0
Macon 68 36 52 31 / 90 40 10 0
Rome 45 24 46 26 / 100 10 0 0
Peachtree City 60 29 49 28 / 100 10 0 0
Vidalia 78 39 49 33 / 60 60 20 5
Winter Weather Advisory until 10 PM EST this evening for the
following zones: Catoosa...Chattooga...Dade...Dawson...Fannin...
Winter Weather Advisory from 10 am this morning to 10 PM EST
this evening for the following zones: Lumpkin...towns...Union...