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Area forecast discussion
National Weather Service Peachtree City Georgia
725 am EDT Thu Apr 27 2017

Previous discussion... /issued 350 am EDT Thu Apr 27 2017/

Short term /today through Friday/...
a squall line continues to move through western and central Alabama early
this morning. This line of storms is ahead of a cold front currently
across central MS.

The line of storms is currently being handled pretty well by the
hrrr. So have followed the hrrr timing and trends fairly closely.
Current surface analysis shows the strongest storms are hugging the
best instability...which is across far southeast MS and roughly the
southern half of Alabama. Expect this trend to continue for the next few
hours. The hrrr does weaken the initial line (the squall line) as it
approaches the County warning forecast area around sunrise and fires additional convection
along the actual cold front as it moves across the County warning forecast area during the
afternoon hours.

Trying to suss out the best areas for strong/isolated severe
convection will be a bit tricky today. The mid level forcing with
this system is expected to get shunted off to the north by mid
morning. So, portions of extreme northern Georgia should see the best mid
level forcing early today. Mid level lapse rates remain marginal for
the entire County warning forecast area after 12z. The best shear is also expected across
northern Georgia...with some decent values south of Interstate 20 this
afternoon. The issue will be instability. The strong/severe
convection across Alabama/southeast MS should continue to hug the best
instability down south through the early morning hours. However, a
few strong (potentially isolated severe) storms could also develop
across far northern Georgia this morning before the squall line weakens.

For late morning and afternoon...the best chances for any strong to
isolated severe thunderstorms will be along the frontal boundary.
With the early morning convection, thicker cloud cover may inhibit
much of northern Georgia and portions of western Alabama from destabilizing as
the front moves through. However, do think areas along and east of
Interstate 85 and south of Interstate 20 should have the best
chances for seeing breaks in the clouds, resulting in good surface
instability this afternoon. With the aid of forcing along the front,
this should be where the strongest convection occurs. By this time,
the best mid level forcing and lapse rates have moved off to the
north and shear values are marginal. So, a few strong storms are
likely with some isolated severe storms possible for the afternoon
hours...mainly east of I-85 and south of I-20.

The models are still progging the frontal boundary to stall across
central portions of Georgia. Enough moisture and some weak frontal
forcing should allow for some isolated storms to develop during the
day on Friday.


Long term /Friday night through Wednesday/...
the long term period will begin fairly tranquil on Friday night.
From a synoptic standpoint...the lower levels will feature a warm
front shifting north of the region and ushering in a pretty moist
and unstable airmass /perhaps most so this season thus far/. This
will be aided in part by the anticyclonic flow from Bermuda sfc
high across the central Atlantic. Pops look to be dying off by
this time with best shot at any lingering storms up in the mtns
where upslope flow would provide best forcing. In the mid
level...things are a bit more interesting upstream from US. A
closed low will have developed across the southern rockies and
will be in the process of inducing sfc cyclogenesis across the Texas
Panhandle area. As this trough digs it will create a downstream
effect which in our case will build a mid level subtropical ridge
into our region from the Caribbean.

With a subtropical ridge influence on will be
hot...thats a given. But despite the synoptic scale descent
associated with the ridge...a very moist /and unstable/ airmass
in place in low levels will allow for the development of diurnally
driven thunderstorms Sat afternoon and evening. Weak shear under
the ridge argues for pulse-severe threat /wind and hail/ given
modest mid level lapse rates and pretty impressive cape. Not
seeing any place to pinpoint best coverage right now given lack of
forcing...but orographic lift within a southerly low level flow
regime in the mtns could help enhance coverage up north. Thinking
this same rule will apply for Sunday...although ridge will shift
east so could see better diurnal storm coverage.

Sunday night into Monday could be interesting as a cold front
approaches the area from the west. This frontal system will be
associated with the aforementioned closed low over The
Rockies...which by this time will have pivoted into the Midwest.
Ingredients do not look quite as good as they once did. Concerned
about the large displacement of upper forcing...along with the sfc
low undergoing occlusion processes by the time the front
approaches the far southeast states. Couple those factors with an
overnight arrival...and confidence becomes hindered on how well
linear mesoscale convective system will sustain itself as it gets close to the state line.
Instability begins to wane /tho non-zero/ by 12z though shear and
helicity environments are impressive just ahead of the line.
Better cape is possible across central Georgia later in day so
could end up having a lower threat across northern and western GA
in the morning with an increasing threat across central GA in the
afternoon. Overall...GFS currently showing greater severe
potential than ec but this is based on slight timing differences
to some extent. This system certainly bears close attention...and
a least a marginal severe threat seems likely at this point. Storm Prediction Center
already highlighting Alabama/MS in a severe outlook.

Conditions look dry behind the front on Tuesday...but broad mid
level cyclonic flow development by Wednesday points to a wet
period beginning at the end of this long term forecast period.



12z update...
squall line ahead of frontal boundary has mostly weakened as it
has approached western Georgia. Do expect additional develop along the
actual cold front later this morning into the early afternoon
hours. Behind the front, cigs will temporarily improve to VFR
through the evening hours. However, IFR cigs expected towards
sunrise on Friday. A wind shift to the SW is expected around

//Atl confidence...12z update...
high confidence all elements.


Preliminary point temps/pops...
Athens 77 62 84 65 / 70 40 20 10
Atlanta 77 63 83 68 / 70 20 20 5
Blairsville 73 52 80 62 / 80 20 20 20
Cartersville 78 58 83 66 / 80 20 20 10
Columbus 80 67 86 68 / 70 40 20 5
Gainesville 75 59 81 65 / 80 30 20 10
Macon 83 65 86 65 / 50 50 20 5
Rome 78 55 84 66 / 80 10 20 10
Peachtree City 78 60 83 64 / 70 20 20 5
Vidalia 88 68 90 70 / 20 20 10 10


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