Scientific Forecaster Discussion
fxus63 klot 300211 aaa
Area forecast discussion...updated
National Weather Service Chicago/Romeoville, Illinois
911 PM CDT Mon may 29 2017
910 PM CDT
A few showers on the scope this evening along with one gusty
thunderstorm to the immediate north of the area passing over part
of the Milwaukee Metro as of 910 PM. These are associated with
one of several ripples that are rotating around the closed stacked
low to the north. This wave was able to catch just enough of
daytime instability and a well-mixed boundary layer as observed
on the 00z dvn sounding to trigger some showers which likely will
fester as they move east through 1-2 hours. Any of these could
bring brief 35 mph gusts, mainly the showers moving into McHenry
and lake counties through 1030 PM.
Have extended pops to handle this and made only minor other
236 PM CDT
Latest radar imagery depicting scattered precip continuing to
develop across portions of WI this afternoon, tied to several
vorticity maxima rounding the base of the large upper level system
to the north. Still anticipate this development to work its way
south into northern Illinois, once again likely staying tied to
the WI/Illinois border. Another area for possible development this
afternoon is across east central Illinois and Northwest Indiana,
along/ahead of eastward moving boundary and surface trough.
Highest chances for widespread development look to be just to the
southeast though, with any precip across the southeast County Warning Area likely
remaining isolated/brief this afternoon. Weak instability still in
place across the entire region, due to rather steep low level
lapse rates. This is still supportive of possible thunderstorm
development this afternoon, although still expected to be
isolated. Any remaining precip/clouds late this afternoon will
quickly diminish this evening.
Similar pattern on Tuesday supportive of additional precip
chances. It does appear though that slightly stronger forcing will
slide further to the south through midday and the early afternoon
time frame. Did increase pops Tuesday afternoon for this possible
scattered development and with weak instability once again
possible, continued slight chance thunder wording.
227 PM CDT
Tuesday night through Monday...
Showers diminish and push east Tuesday night as a secondary cold
front and the upper level vort streamer move through. High pressure
slides over or just south of the forecast area Wednesday. With ample
sunshine, raised high temps a couple of degrees Wednesday.
Active weather returns Thursday and appears to last through the
weekend. Temps and dewpoints increase Thursday. Thursday could be
warmer than currently forecast as local guidance suggests 80s are
possible, but with clouds and precip in the forecast, unsure on how
well we will warm. The GFS and NAM both have a weakening mesoscale convective system
reaching the forecast area Thursday afternoon. The GFS is much more
aggressive with precip than the other models, but a lingering mesoscale convective vortex
could lead to new convection in the region. As such, left a chance
for showers and storms Thursday afternoon. Precip should become more
widespread Thursday night as a low level jet points into the region.
As the previous discussion suggested, main threat will probably be
heavy rainfall as precipitable water values are around 1.25 inches, which is above
average. In addition, steering flow is weak so storms will likely be
Showers and storm persist through Friday. A backdoor cold front
slides down the lake and will provide additional forcing. Forecast
soundings feature 1000-1500 j/kg of cape, but shear is weak. Precipitable water
values remain above average so main threats will likely be heavy
rain and maybe hail.
Initially thought Saturday could be dry like the GFS suggests, but
the 12z European model (ecmwf) came in with a stalled front and precip. Given
northwest flow aloft, kept slight chance precip chances in the
forecast. The same is true for Sunday, models differ on if there
will be precip, but with northwest flow, I do not feel comfortable
going dry in the extended forecast. Models agree on high pressure
early next week so went dry for Monday.
for the 00z tafs...
623 PM...gusty westerly winds will diminish with sunset and
likely remain under 10kts overnight. Speeds/gusts will increase
again on Tuesday morning with westerly winds remaining gusty
through sunset Tuesday evening.
An isolated shower is possible this evening...mainly near the
Illinois/WI state line but most of the area will remain dry. As an upper
low and a bit cooler air aloft rotates closer to the area Tuesday
afternoon...isolated/scattered showers are expected during the
mid/late afternoon Tuesday and may linger into the early evening.
An isolated thunderstorm is also possible but expected coverage
too low for mention this far out and will continue with prob
shower mention. Cms
227 PM CDT
Gusty west winds diminish and will no longer pose a threat to small
craft this evening. Low pressure will slowly shift northeast from
central Ontario to James Bay through Wednesday morning, and a high
will build over the plains through Tuesday night. As such, an
extended period of westerly winds is expected through Wednesday
night. The high passes south of the lake Wednesday night. A trough
of low pressure shifts from western Ontario Thursday morning to the
northern Great Lakes Friday morning. A cold front will shift south
down the lake Friday and winds become north behind it. High
pressure then builds over the western Great Lakes early next week.
Visit US at http://weather.Gov/Chicago (all lowercase)