Forecast-Discussion

000
FXUS63 KLOT 231443
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
943 AM CDT Thu Mar 23 2017

.UPDATE...
934 AM CDT

Concerns today include chances of showers late this afternoon/this
evening, and even hints at some thunder potential.

Latest RAP/HRRR and 12z NAM guidance depict that current dry air
mass over the area will lead to shower activity struggling to
really get going, other than some sprinkles. Once the lower
moisture gets in place during the warm frontal passage, we lose
the deeper moisture over Iowa which appears to move through in
the mid-late afternoon. There are some pretty steep lapse rates
upstream that advect eastward with the elevated warm layer
depicted well on KABR and KOAX 12z upper air soundings with
700-500 mb lapse rates in excess of 7-7.5 deg/km. Have not
introduced thunder chances here given the concerns mentioned above
about how much we will see anyways given the narrow corridor of
forcing ahead of the warm front. But the time period of interest
for any activity would be in the 5pm- 9pm time frame. Storms
remain possible over the lake briefly this evening.

KMD

&&

.SHORT TERM...
330 AM CDT

Through Friday...

Lots to talk about this morning, in the short term, precip
chances, temps, and cloud cover are the biggest concerns.

Have continued the trend of lowering pops for today as low level
dry air and weakening of approaching shortwave both look to pose
impediments against more robust precip chances. Still concerned
that there could be a shower or sprinkle, but chances of anything
measurable this afternoon and evening appear low.

Extensive cloud cover upstream is moving into the area and
expected fairly cloudy skies today. Significant low level warm
air advection doesn`t arrive until tonight, so with cloud cover
today have trended highs toward the lower side of guidance.

Developing warm front will lift north this evening, which should
shift the focus of nocturnal convective/precip chances well north
of our cwa. In addition, strengthening southerly winds should
begin to draw warmer and more moist air north into the area.
Evening lows could dip into the upper 30s/low 40s, but expect a
fairly significant rise in temps through the night with temps in
the 50s by sunrise Friday morning.

Temperature forecast is tricky Friday and bust potential is quite
high. There are essentially 2 different lines of reasoning, the
first being the domestic models (NAM & GFS) which suggest low
stratus will blanket the area and the NAM keeps highs in the 50s
and GFS only in the 60s. Conversely, there is the foreign model
camp (ECMWF & GEM) which keeps the area largely stratus free
Friday and sends temps soaring well into the 70s, even upper 70s
for Chicago in the GEM. The ECMWF has very strong support from its
50 member ensemble with very minimal spread.

Using backward trajectories in the models, the low levels (

NWS LOT Office Area Forecast Discussion