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fxus63 klot 100016
afdlot
area forecast discussion
national weather service chicago/romeoville il
616 pm cst tue feb 9 2010
. discussion...
306 pm cst
short term ... tonight and wednesday...
have decided to drop blizzard watch across southeastern cwa due to
concerns about meeting strict criteria. other than minor tweaks to
snow amounts have maintained short term forecast pretty much as is
through wednesday.
afternoon satellite imagery shows stout upper low near stl ... which
will continue to move east/southeast across ohio river valley
tonight. light/occasionally moderate snow continues in broad
cyclonically curved band of isentropic upglide from southern ohio
valley and lower great lakes into wi/northern il/ia. as mentioned in
midday afd update ... areas to watch for enhanced snowfall are across
counties bordering lake michigan where combination of low level
convergence and steep lapse rates helping to maintain stronger
reflectivities and lower observed visibilities ... and across portions
of i - 80/i - 88 corridors over western counties where low level
frontogenesis maxima occurring north of sharp low level baroclinic
zone associated with sfc trough axis. radar trends appear to confirm
idea of lake enhanced snowfall affecting portions of northeast
il/chi metro area through the remainder of the afternoon/evening
rush hour ... with moderate/heavy snow at times currently over
northern suburbs gradually shifting south across the city by evening
and into northwest indiana overnight. recent report of 7 inches in
lake county il suggests going 8 - 12 inch storm total for lake
michigan counties still appropriate. have made only minor tweaks
elsewhere with strengthening deformation/frontogenetic band to
supply several more hours of snow to remainder of forecast area into
this evening.
evening phase of storm will be accompanied by increasing
north/northwest winds tonight as sfc low continues to deepen for a
time off to our east. sustained winds approaching 20 kts with gusts
near/around 30 kts expected overnight especially across southeastern
portion of cwa ... with blowing and drifting of snow expected to
become widespread across the area tonight. while conditions suggest
we may approach blizzard conditions in a few spots at times...
confidence in meeting blizzard warning criteria of 3 hours of 35 mph
and 1/4sm visibility not high enough to upgrade to blizzard
warning ... especially with slightly lower snow/liquid ratios earlier
in the day and snow winding down as winds really pick up tonight.
therefore will let winter storm warning ride with emphasis on
blowing/drifting and poor visibility.
lingering lake effect into porter county late tonight/wednesday
morning ... though forecast soundings over southern lake michigan
indicate inversion heights/equilibrium levels decreasing rapidly
during the morning wednesday. at this time have maintained snow
amounts from previous forecast for lake snow belt areas ... with 10 to
15 inches across northeast porter county by the time it tapers off
wednesday afternoon.
ratzer
long term ... wednesday night through tuesday...
at 500 mb ... a northwest flow will continue over northern illinois
and northwest indiana wednesday night through friday. a 500 mb
trough is forecast to move into minnesota by friday evening. this
trough will move through illinois friday afternoon. we will
forecast a chance of snow friday. another 500 mb trough is
forecast to move through illinois late saturday night and sunday
morning. this trough is forecast to be more intense than the one
that occurred friday. a surface low will develop in iowa saturday
evening and will move across central illinois sunday. then by
sunday evening the surface low will be in southern ohio. another
high will move into the mississippi valley monday night. we used
the gfs model for guidance and for temperatures ... we followed mos
guidance. for the last seven days the mean absolute errors were
lower than the gfs mos guidance. the forecast high temperatures
will be in the middle to upper 20s thursday through monday. the
forecast low temperatures will be in the teens saturday through
tuesday.
ww
&&
. aviation...
615 pm cst
0000 utc tafs ... ifr/lifr to prevail this eve with increasing lake
effect snow and development of blsn as winds gust to around 30kt.
conditions to improve to mvfr during early morning - predawn hours
as lake effect shifts into nw ind and wind gusts drop off a bit
lessening tempo vsby restrictions in blsn.
satellite and profiler winds show main upper low over northeast
il at 23z while 500hpa low moving from far southern il into ky. as
whole system continues to move east over the midwest and oh valley
this evening and overnight the synoptic snow will continue to
taper off and shift east. however ... with system moving to the east
of the local area the low and mid level flow has been backing to
ne - nne and mid levels cooling significantly. in response ... lake
effect has been gearing up as fetch continues to increase and base
of inversion lifting to around 2km agl with moist adiabatic lapse
rates from surface to inversion base.
radars show continued increase in number of moderate to heavy snow
shower cells withing the light synoptic snow ... with the cells
gradually shifting further south across northeast il. 18z nam12
had good depiction of lake effect setting up so have generally
followed it in developing a main convergent band from mid lake
over central lake mi ssw to the southeastern wi shore and across
far ne il and nw ind as the evening progresses.
from what has been observed upstream into se wi where these cells
have been occurring for a while now vsby drops to 1/2 - 3/4sm range
as the pass over ... with an occasional 1/4sm when core of heaviest
cells pass right over obs site.
anticipate transition from cellular to multi - band to single band
lake effect with time as flow continues to back to the n and
eventually low level winds go to northwesterly. heaviest lake
effect expected this eve with band to have shifted to along il - in
border by 06z and into far nw ind by 09z.
trs
&&
. marine...
139 pm ... primary forecast concern remains gales thru wednesday
evening.
overall ... no major changes to going forecast/grids. a large ridge
of high pressure extends from hudson bay southwest across the
northern and central plains. this ridge will weaken as it slowly
shifts east wednesday and wednesday night and becomes centered
over the ohio river valley thursday night. ahead of this ridge is
an area of low pressure lifting northeast across ohio. this low
will deepen further as it reaches lake erie this evening ... then
new low pressure will develop off the mid atlantic coast and
rapidly deepen tonight ... absorbing the low over lake erie.
as a result ... northeast winds will continue to increase thru this
evening increasing to gales. winds will turn more northerly after
midnight then turn northwest wednesday morning. the gradient will
slowly weaken ... especially after the low over lake erie weakens
and is absorbed. thus gales will diminish wednesday afternoon
into wednesday evening. but since the high will re - center itself
over the ohio valley ... the gradient will remain tight across the
western lakes through thursday morning ... thus winds will remain
strong wednesday night.
still some uncertainty regarding the next system to affect the
lakes region this weekend. low pressure is expected to move
across the southern lakes region with another strong high
building across the plains. however ... wind speeds will be
dependent on the strengthen/location of the low and with low
confidence from this distance ... will keep winds in the 15 - 25kt
range for now. cms
&&
. lot watches/warnings/advisories...
il ... winter storm warning ... ilz003 - ilz004 - ilz005 - ilz006 - ilz008 - ilz010-
ilz011 - ilz012 - ilz013 - ilz014 - ilz019 - ilz020 - ilz021 - ilz022-
ilz023 - ilz032 - ilz033 - ilz039 until 9 am wednesday.
in ... winter storm warning ... inz001 - inz002 - inz011 until noon wednesday.
winter storm warning ... inz010 - inz019 until 9 am wednesday.
lm ... gale warning ... lmz080 - lmz669 - lmz671 - lmz673 - lmz675 - lmz743 - lmz744-
lmz745 - lmz777 - lmz779 - lmz870 - lmz872 - lmz874 - lmz876 - lmz878
until 9 pm wednesday.
gale warning ... lmz261 - lmz362 - lmz364 - lmz366 - lmz563 - lmz565 - lmz567-
lmz740 - lmz741 - lmz742 - lmz868 until 4 pm wednesday.
&&
$$