518 
fxus64 kjan 210927 
afdjan 


Area forecast discussion 
National Weather Service Jackson MS 
327 am CST Sat Nov 21 2009 


Short term...today through Sunday night...primary forecast concerns 
for this period will be rainfall associated with slowly deepening low 
pressure over the Gulf...along with the associated winds. Early 
morning surface analysis places a 1008 mb surface low over the 
northwest Gulf of Mexico...about 120 miles southeast of Houston 
Texas. This low continues to develop as vigorous deep-layer ascent 
downstream of a shortwave trough embedded within the southern jet 
stream continues to interact with a baroclinic zone over the western 
Gulf. Among multi-model guidance...the GFS/NAM/sref/UKMET appear to 
initialize the depth and position of the low quite well. By this 
evening...the NAM shows the low assuming more of a northward heading 
while elongating the low from northwest to southeast...and following 
a track similar to the European model (ecmwf). Tight clustering amongst GFS ensemble 
members around the operational GFS/UKMET/sref track solutions gives 
higher credibility toward these solutions...and thus the more 
southern low track is the preferred solution. This calls for the low 
tracking east-northeastward south of the Gulf coastline through 
Sunday morning before making landfall over the western Florida 
Panhandle. 


Given the more southern low track...the northward extent of precipitation 
coverage today is in question...and local high-res WRF ensemble 
members that use preferred GFS parameters as input suggest that rain 
coverage across northern areas will not be as widespread as 
previously anticipated. Thus...probability of precipitation were trended downward to likely 
over the northern third of the County Warning Area...consistent with latest mav 
guidance. However...all WRF members indicate measurable precipitation 
occurring along and south of Interstate 20...where probability of precipitation were 
increased above mav guidance to 100 percent for today. The other 
implication of the more southern low track is that the probability of 
meeting lake Wind Advisory criteria on Ross Barnett Reservoir and 
Okatibbee Lake has decreased. The total south-to-north pressure 
difference across the County Warning Area is not expected to exceed 3.5 mb today or 
tonight...supporting sustained northeast winds of 10 to 15 miles per hour on the 
lakes. Forecast soundings from the GFS indicate maximum flow below 700 mb 
remaining at or below 25 knots...and even winds this strong will 
struggle reaching the surface as gusts given stable thermal profile 
below 700 mb. Gusts are expected to remain below 25 miles per hour. In fact...WRF 
ensemble now indicates less than 15 percent chance of lake Wind 
Advisory criteria being met on the lakes...and thus the advisory has 
been cancelled. The stronger low-level wind field suggested by the 
NAM appears unlikely given the anticipated southern low track. 
Additionally...elevated instability supporting MUCAPES from 200 to 
500 j/kg will affect the southeastern third of the forecast area this 
afternoon and evening...where isolated thunderstorms are possible. 
Near zero sbcapes will preclude presence of surface-based convection 
or risk for strong thunderstorms. High temperatures today mostly 
followed mav guidance...though they were raised a couple degrees 
across northern areas...where onset of rain will be the latest. 


For tonight...rain will continue to spread across the eastern half 
of the County Warning Area...where probability of precipitation have been raised above mav guidance to 100 
percent with high confidence of measurable precipitation from the sref. 
Total rainfall amounts through tonight of one-half to one inch are 
expected...which will be too low for flash flooding. Meanwhile... 
drier air wrapping into the southwestern semicircle of the low will 
limit precipitation chances over the southwestern County Warning Area...where rain is 
expected to end during the evening. On Sunday...the surface low will 
begin filling as the supporting middle/upper-level shortwave trough 
becomes absorbed into the large-scale jet stream and decouples from 
the surface circulation. As such...precipitation chances and wind speeds 
will decrease...though enough moisture west of the low will support 
a chance for rain over eastern areas Sunday morning per mav 
guidance. By Sunday evening...most of the rain is expected to have 
tapered off. Skies will begin clearing from southwest to northeast 
on Sunday...and thus temperatures were increased a couple degrees above mav 
guidance across the southwest. Then for Sunday night...clearing 
skies combined with the moist ground will set the stage for patchy 
fog development...particularly across the southern portion of the 
County Warning Area where highest rainfall totals through Sunday are expected. 
/Cohen/ 


Long term...a rather flat zonal flow pattern will transition back to 
ridging in the western Continental U.S. And troughing in the east during the coming 
Holiday week. The primary feature for the week appears to be a weak 
frontal system passing through the lower MS valley Tuesday and 
Tuesday night with minimal rainfall chances. This boundary is 
followed by two reinforcing troughs Wednesday night and later on 
Saturday to keep US cooler than climatology. 


Quite mild conditions begin the week on Monday with foggy lows in 
the 40s and highs recovering nicely into the M-u60s with considerable 
sunshine. The GFS is now mustering up 0.75-1.0 inch precipitable waters  along the 
Tuesday front before returning to 0.25 inches on Wednesday. Pre 
frontal low level ridging along the Gulf Coast should prevent much 
moisture return and do not see much moisture convergence with the 
boundary so will keep mention to light rain. The European model (ecmwf) is actually a 
little wetter with this front as it shows a tenth of an inch qpfs 
hanging on into Tuesday evening over the southeastern portions of our County warning forecast area. 
Beyond that...a quick reinforcing dry trough sweeps through the area 
with a few clouds for Wednesday night. This sets up a chilly cold air advection 
celebration for Thanksgiving day with most lows in the u30s and highs 
in the 50s. 


A 1020+mb surface high over southeastern Texas Friday moves along the Gulf Coast 
for Friday night and Saturday with some moisture return for Saturday 
night. Another weak front could reach the area by that time but it 
also is appearing rather dry with the surface high off the coast of 
northwest Florida. Will have to monitor this time frame as the European model (ecmwf) again a 
little more robust with the mass fields and higher rain chances. 


All in all...the Holiday week should be quite dry and also seasonable 
in the temperature department. The ensemble MOS guidance stays within 
1-2 Standard deviations and in most cases the means are close to the 
operational numbers./40/ 


&& 


Aviation...rain will spread from southwest to northeast across the 
area today...as an upper level disturbance and surface low pressure 
approach the region. Rainfall intensity is expected to be light to 
moderate...with the greatest coverage of rain across central and 
southern Mississippi today. For tonight...the greatest coverage of 
rain will be across eastern Mississippi. MVFR conditions due to 
ceiling restrictions and occasional visibility restrictions will 
affect much of the area by this afternoon...with occasional IFR 
ceilings and visibilities possible at khbg and kmei near the moderate 
rain. An isolated thunderstorm will also be possible across southeast 
Mississippi this afternoon and evening...though the chance of 
thunderstorms occurring at any taf site is too low for mention in the 
tafs at this time. Northeast to east winds are expected to increase 
to 7 to 12 knots by this afternoon...but will slowly diminish late 
tonight. /Cohen/ 


&& 


Preliminary point temps/pops... 
Jackson 57 49 61 45 / 100 83 15 9 
Meridian 57 47 62 44 / 100 98 35 15 
Vicksburg 58 47 62 42 / 100 48 15 7 
Hattiesburg 59 49 64 46 / 100 96 15 9 
Natchez 56 48 63 43 / 100 47 10 3 
Greenville 59 47 62 44 / 59 50 15 7 
Greenwood 60 47 61 44 / 62 89 30 10 


&& 


Jan watches/warnings/advisories... 
MS...none. 
La...none. 
Arkansas...none. 
&& 


$$ 

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