Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1121 PM CST Thursday Mar 5 2015
306 PM CST
There are no weather concerns through the forecast period...unless
you are concerned with the arrival of milder weather. High pressure
over the area will drift southeast tonight but not before clear
skies and light winds conspire to bring one last night of near to
below zero temperatures away from Chicago. Temperatures should
steady or slowly rise overnight except closer to the surface ridge in
the southern County Warning Area...where temperatures may still be below zero
Northwest flow aloft will remain on Friday...but downsloping of
milder air from High Plains will shunt cold air mass off to the
east. Tight pressure gradient between high pressure departing to
southeast and low pressure to the north will result in gusty
southwest winds. So after chilly start to the day...expecting
highs in the upper 20s to lower 30s. Did trend closer to warmer
side of guidance based on climatology of expected 850/925 mb temperatures by
the afternoon. Would not be too surprised if Chicago heat island
tags the middle 30s. Middle/upper disturbance will bring plenty of middle
and high clouds across the area and gusty winds will keep a raw
feel however. Steady southwest winds ahead of a weak cold front
will keep lows in the 20s Friday night...then the front will cross
the area on Saturday. Have maintained a dry frontal passage. 850 mb temperatures
and 925 mb temperatures within a degree or so of 0 celsius support upper
30s to middle 40s for highs. The glacial snowpack should begin
melting in earnest on Saturday after likely some minor melting on
329 PM CST
Saturday night through Thursday...
The big story next week will be the likely Spring like
temperatures on Tuesday through Thursday. Some model differences
on Sunday...with 12z European model (ecmwf) and Canadian global model bringing a
clipper type system just north of the County Warning Area and light precipitation to the
northern County Warning Area in morning and afternoon...while GFS is slower with
weak shortwave along weak cold front during the evening. Have gone
ahead with mainly slight probability of precipitation on Sunday. If precipitation does occur...wet
snowflakes may mix in with spotty light rain.
Cold air will be non existent behind the cold front and
progressively warmer air will be advected into the area through
middle week next week around periphery of expansive high pressure to
our southeast that will also keep things dry. 500 mb heights will
likely approach late Spring like ~570 dm levels by Thursday.
Looking at highs possibly well into the 50s on Tuesday and on
Wednesday...global guidance progging high single digits celsius at
850 mb and possibly around or a bit over 10 celsius at 925 mb.
Since snowpack is likely to have been reduced to remnant piles at
that point...it is conceivable that highs could be in the 60s for
much of the area as shown on 12z European model (ecmwf). For now...have
conservatively gone above guidance blend with middle-upper 50s highs
on Wednesday. Potential exists for a backdoor cold front late next
week...but if timing is slower...Thursday could be as warm as if
not warmer than Wednesday. Due to overall lack of precipitation
expected with coming warmup...snow melt should be gradual and not
expecting significant Hydro issues...but rises are possible on
more ice jam prone rivers.
//Ord and mdw concerns...updated 06z...
* gusty SW winds with maximum gusts up to 25 knots in the afternoon.
* MVFR ceilings are possible Friday evening.
High pressure is slowly shifting southeast and winds will become
southeast to south-southeast overnight. The gradient tightens tomorrow with south-southeast winds
gusting to around 21 knots by 14z. Gusts increase to around 24 knots by middle
day as winds veer slightly to southeast. A weakening surface trough and
boundary move through the region Friday evening and a fair number
of guidance members feature MVFR or IFR ceilings with the trough. Have
limited confidence in the ceilings as guidance can be over zealous
with low level moisture...but put a scattered MVFR ceiling in the taf. Southeast
winds diminish to around 10 knots in the evening.
//Ord and mdw confidence...updated 06z...
* low confidence in MVFR or lower ceilings developing Friday evening.
* High confidence in all other forecast elements.
//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...
* Saturday...dry/VFR. West winds.
* Sunday...slight chance of rain or snow. Southwest winds.
* Monday through Wednesday...dry/VFR. West/southwest winds.
147 PM CST
High pressure remains centered over the Ohio Valley this
evening...but will drift east overnight and settle across the
southeastern states through the weekend. Flow across the lake will
remain generally from the west to southwest...with a slightly
better gradient across the northern half of the lake. The gradient
is poised to increase Friday morning with the approach of a low
pressure system into the western Great Lakes and the departure of
a surface ridge to the southeast. Winds will be nudging up Friday
with winds approaching 30kt...and possibly a few gale force gusts
Friday midday through early evening. The winds will diminish
Saturday morning...then with a series of weak low pressure systems
poised to move across over the next several days with flow mainly
from the west to southwest.
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...9 am Friday to 4 am Saturday.
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