Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1239 am CDT Tuesday Apr 21 2015
248 PM CDT
Anonymously low middle-level heights...associated with a large and very
slow moving middle/upper level low across the north central
Continental U.S....will result in a prolonged period of below to much below
normal temperatures across much of the Great Lakes region. As
such...the main forecast concerns during the short term period will
therefore largely revolve around temperatures and also winds over
the next 24 to 30 hours. It appears we will have a decent potential
for some areas of northern Illinois to approach the freezing mark
for low temperatures by mid-week...so frost and freeze issues could
Satellite imagery this afternoon looks more like its late October
rather than late April...with cold season closed cellular cumulus
evident over the region. Temperatures across the region also
resemble fall-like conditions with temperatures under the cloud
cover struggling to make it near 50 degrees. Then of course the
gusty westerly winds up around 30 knots...certainly makes it feel even
colder then the current temperatures suggest.
Although wind gusts will ease this evening...wind speed will
increase again on Tuesday. A large lower and upper low will remain
nearly stationary across the upper Midwest and southern Ontario into
Tuesday...and this will continue to result in Stout pressure
gradient across the area...supporting another windy day. It appears
that wind gusts Tuesday afternoon could be several knots stronger
than they were today...as deep atmospheric mixing over 8,000 feet
above ground level is expected. With winds of 50 to 60 knots at this heights...a
percentage of this magnitude will likely be mixed towards the
surface. This looks to result in 35 to possibly 40 knots wind gusts
over the area. Because of this...a Wind Advisory may be needed for
Tuesday afternoon. However...after collaborating with surrounding
offices...we have chosen to let the midnight shift get another look
at things before locking into a Wind Advisory headline. Either way
it will be a windy day Tuesday.
In addition to the winds...a few scattered showers are possible
later Tuesday afternoon...mainly northern areas. Several small scale
disturbances are expected to rotate east-southeastward through the
larger scale middle/upper level low over the upper Midwest. The
combination of this and steep lower level lapse rates due to the
unseasonably cold air aloft...could support a few afternoon showers.
It these showers do indeed develop far enough south into northern
Illinois the low freezing levels and steep lapse rates through the -
10 to -20 c levels could result in some graupel reaching the
surface...and or an isolated lightning strike. Though confidence in
this is low.
There appears to be a slightly better chance for some rain showers
Tuesday night...mainly across my southern areas as yet another middle
level disturbance shifts over the area. This could interact with the
lower level baroclinic zone over central portions of the state to
result in a band of enhanced fgen...and hence a focus for forced
ascent over southern sections of the area to support some rain
showers. At this time I will add in some low end probability of precipitation across my
southern County Warning Area to cover this threat.
For Wednesday and Thursday we look to continue under this abnormally
chilly weather pattern. However...the good news is we will likely
see more sun and lighter winds...especially by Thursday.
Therefore...it should not feel quite as cold as today. At night
though we could be looking at near freezing conditions by Thursday
morning...though wind speeds may not drop off quite enough to
result in widespread frost until Friday morning. This will need to
248 PM CDT
Thursday night through Monday...
Unseasonably cool temperatures and periodic showers persist through
the weekend as the upper low and its surface counterpart continue to
spin over Quebec through the end of this week and into at least the
early part of next week. With split flow across the western
states...our best chance of precipitation through this period appears to be
focused in the southern portion of the forecast area as shortwaves
get ejected from the southern stream trough. Ensembles have a
different handling of this energy than do the deterministic
solutions...and a deterministic solution like the European model (ecmwf) has its
shortwaves quite a bit stronger and slower and farther north than
the other models...but all agree that the strongest wave reaches the
area sometime on Saturday. This would be the same system outlooked
by Storm Prediction Center to bring a threat of severe weather to the western Gulf on
Friday...and then prognosticated by the models to bomb out along the
Atlantic coast late in the weekend as it gets pulled into the trough
of the Quebec low.
Confidence is not enough to go with more than chance probability of precipitation on
Saturday...and no other period has enough consistency in the
guidance to include mentionable probability of precipitation at all...though this does not
necessarily mean the entire extended period will remain dry. But
even in the most northern solution as shown by the European model (ecmwf)...Gulf
warmth and moisture remain cutoff to the south...so precipitation locally
would likely be in the form of showers rather than thunderstorms.
Fire weather...443 PM CDT
..erratic fire behavior Tuesday...
Strong winds are likely during the day Tuesday with winds expected
to slightly stronger than they were today. Gusts tomorrow afternoon
are expected to be in the 40 to 45 miles per hour range. Deep mixing into a dry
airmass will result in effective mixing of dew points during the
day. Current projections are for afternoon minimum relative humidity values to fall
into the 25 to 35 percent range however several models hint that we
may trim an additional few degrees off of dew points resulting in relative humidity
values closer to 20 percent.
Winds will likely exceed red flag warning criteria and min relative humidity values
will be in the ballpark of red flag warning criteria...however given
yesterdays half to three quarters of an inch of rain...10 hour fuel
moisture may not reach red flag criteria. Faster drying fuels
however will likely dry out quickly given the conditions tomorrow
resulting in an elevated fire danger.
//Ord and mdw concerns...updated 06z...
* strong west winds...potentially gusting to or just over 40kt
at times late Tuesday morning through the afternoon
No real change in this set of tafs. Expect winds to rapidly
increase during the day Tuesday...especially middle to late morning.
Current tafs reflect what would generally be the expected steady
state conditions with gusts 35 knots (higher at gyy)...but gusts at
most terminals could reach 40 knots at times. Expect a rapid decline
at sunset with a slow shift from a 270 direction to more of a northwest
wind by daybreak Wednesday. VFR ceilings expected Tuesday afternoon
with maybe a passing and short lived sprinkle or shower. A more
a quick moving and more organized band of rain will pass south of
the main terminal areas Tuesday late evening and overnight.
//Ord and mdw confidence...updated 06z...
* high confidence in all forecast elements...except medium-high
confidence on strength of forecast gusts which could
occasionally be slightly higher.
//outlook for Ord/mdw for 12z Wednesday-12z Monday...updated 00z...
Wednesday through Sunday...primarily VFR.
304 PM CDT
Westerly offshore winds continue to gust up around 30 knots this
afternoon over the near shore waters...and these winds will be slow
to diminish this evening. Another windy day is on tap for
Tuesday...and magnitudes look to be stronger than they were today.
Therefore...it appears that gales of 35 to 40 knots will be a decent
bet along the Illinois and Indiana near shore waters due to the off
shore component. However...over the open waters it appears that the
near surface marine stable layer will not allow these stronger winds
to reach the water surface. Therefore...the gale force winds may end
up only extending off shore for a few miles...with weaker winds over
the open waters. Due to this expectation...a gale watch is being
issued only for the near shore waters for Tuesday.
Lm...Small Craft Advisory...nearshore waters
until 9 am Tuesday.
Gale watch...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745...9 am
Tuesday to 10 PM Tuesday.
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