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fxus64 kjan 172042 
afdjan 


Area forecast discussion 
National Weather Service Jackson MS 
342 PM CDT Wednesday Mar 17 2010 


Discussion... 


Tonight through Friday night... 


The latest satellite imagery shows two upper level lows that are 
influencing arklamiss weather today. One is drifting eastward across Alabama/Georgia 
and the other is digging southward over the Ozark/arklatex region. The 
large stratocumulus deck rotating southward around the back side of 
the Alabama/Georgia low is holding strong and is keeping the air cooler this 
afternoon east of the MS river. Patchy light rain will develop over eastern MS 
due to the westward shift of the deformation axis over Alabama...and this 
should continue through the evening and into the early morning hours. 
Along and west of the MS river...low end probability of precipitation for light rain this 
evening still look reasonable due to the approach of the lift associated with 
the Ozark upper low. Rainfall will be very light where it occurs. 


The latest GFS/gefs/ECMWF/NAM/sref are in excellent agreement in 
showing that the upstream low will be absorbed by the Alabama/Georgia low 
Thursday/Thursday night and this will allow a strong shortwave ridge to 
move into the arklamiss region bringing mostly clear skies by late 
Thursday into Thursday night. Temperatures Friday will be much warmer with 
highs actually above climatic average for a change. Quiet weather 
will continue through Friday night before the weather gets active 
again over the weekend. /Ec/ 


&& 


Long term...(saturday through next wednesday)...the next storm 
system will affect the region late Saturday into Saturday night...as 
a deep- layer trough digs southward in the Lee of The Rockies. The 
trough will initially dig southward so fast while maintaining a short 
wavelength that the southern extent of the trough will close off 
yielding a closed middle/upper-level cyclonic circulation. This feature 
will become somewhat disassociated from a northern stream on 
Saturday...yielding a split flow pattern east of The Rockies. The 
operational GFS appears to maintain the best run-to-run continuity 
with regard to the evolution of this feature...and has the most 
backing from the GFS ensemble members...CMC...and NOGAPS. The European model (ecmwf) 
appears to disassociate the middle/upper-level low too aggressively 
compared to general multi-model consensus...and yields a much deeper 
low at a much more southern latitude than other guidance. The deep- 
layer trough/low will be associated with a surface cold front that 
will sweep across the region mainly after 21z Saturday. The delay in 
the frontal timing relative to previous model runs is due to the 
lagging middle/upper-level support as the circulation closes off and the 
split-flow pattern becomes established. A surface wave/low is 
forecast to form along the front in the middle-south region and ride 
north-northeastward along the front...as deep-layer ascent pivots 
cyclonically around the middle/upper trough that becomes negatively- 
tilted Saturday night. If the low becomes as disconnected from the 
northern stream as the European model (ecmwf) suggests...than an even slower frontal 
passage would be realized with much more vigorous forcing further 
south and a much more quickly-maturing low further to the south. 
However...this solution is currently an outlier. 


Despite the slightly slower frontal motion now prognosticated...this will 
still be a fast-moving system...reducing the amount of time for 
substantial moisture return. Surface dewpoints are only expected to 
climb into the 50s ahead of the cold front...with low- to middle-level 
lapse rates around 6 c/km along and ahead of the front. Latest nwp 
guidance indicates MUCAPE values remaining around or below 
200 j/kg...suggesting little chance for thunder. However...given the 
strength of the deep-layer ascent particularly as suggested by the 
European model (ecmwf)...chance for thunder has been maintained in the forecast for 
Saturday afternoon into Saturday night. Forcing could be strong and 
focused enough for a strongly-forced...low-topped convective line to 
sweep across the region with the cold front and could be capable of 
producing gusty winds as a 50-knots 700 mb jet maximum approaches from behind the 
cold front with a sizable cross-frontal component. Probability of precipitation were 
increased above mex guidance for Saturday night based on consensus 
amongst multi-model ensemble guidance. Also...gusty southerly winds 
ahead of the front could necessitate lake wind advisories for 
Saturday. 


Gusty west to northwest winds will advect an unseasonably cool 
airmass into the region behind the cold front for Saturday night 
into Sunday...and these wind speeds could also reach lake Wind 
Advisory criteria. 1000-850 mb thickness values are expected to fall 
into the 1290-1300 geopotential meter range for late Saturday night. 
Forecast low temperatures were dropped a few degrees below mex 
guidance into the middle and upper 30s for Saturday night west of 
Interstate 55. The potential for a slower frontal passage along with 
wrap-around moisture and associated low-level clouds reduce 
confidence in even colder temperatures. 


High temperatures on Sunday will be 10 to 15 degrees below seasonal 
averages with the cold airmass moving into the region and 850 mb temperatures 
falling to between -2c and -6c. Temperatures Sunday night will be 
the coldest in quite awhile...as the surface ridge moves over the 
County Warning Area and skies clear. This will set the stage for optimal radiational 
cooling conditions...and with dewpoints falling to the upper 20s to 
around 30...freezing temperatures could be experienced across the 
area. This potential for freezing temperatures seems further 
justified by the ensemble GFS and operational GFS indicating the 
Arctic oscillation index falling to around 2 to 3 Standard 
deviations below normal by this weekend...with a well-modified 
Arctic airmass affecting the County Warning Area. The colder airmass will persist 
into Monday night. Thereafter...temperatures will begin moderating 
toward seasonal averages early next week...as the surface ridge 
translates eastward and the airmass further modifies. 


The flow aloft will become increasingly southwesterly for Tuesday 
into Wednesday as a longwave trough becomes established over the 
western Continental U.S.. latest guidance indicates this feature ejecting a 
series of shortwave troughs across the region by the middle of next 
week as moisture returns to the region and instability increases. As 
such...a slight chance for showers and thunderstorms has been 
introduced into the forecast for Wednesday. The overall synoptic 
pattern could become more conducive for deep moist convection by 
late next week as the aforementioned longwave trough makes further 
eastward progress. /Cohen/ 


&& 


Aviation...the disturbed upper level pattern over the region will 
continue to bring mostly cloudy skies for the rest of the afternoon 
through Thursday morning...although most ceilings will be just above 
VFR range. Ceilings at middle afternoon were from 2.5- 6.5 kft. MVFR 
ceilings will still be possible at times...but widespread and 
persistent MVFR ceilings (or tempo IFR ceilings) are really not 
expected at this time. Expect isolated light showers around the 
region this afternoon and tonight. Otherwise...north winds today will 
gust up to 15 miles per hour at times...but will lighten again tonight. /17/ 




&& 


Preliminary point temps/pops... 
Jackson 42 64 40 71 / 11 7 0 0 
Meridian 41 66 38 71 / 18 16 0 0 
Vicksburg 42 65 39 71 / 13 3 0 0 
Hattiesburg 43 65 41 72 / 7 9 0 0 
Natchez 42 64 40 70 / 18 2 0 0 
Greenville 44 63 43 71 / 14 3 0 0 
Greenwood 43 65 39 71 / 17 8 0 0 


&& 


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


&& 


$$ 


Ec/cohen/17 

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