Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
1237 PM CDT Wednesday Jul 30 2014

issued at 1233 PM CDT Wednesday Jul 30 2014

Diurnal clouds have developed as expected but so far no new
convection has developed. Downward motion is still prevailing
across the area which will take time to overcome. Thus isolated
showers and storms will likely hold off until peak heating this

Update issued at 920 am CDT Wednesday Jul 30 2014

Based on recent trends...thermals should be initiated in about 90
minutes. New diurnal convection should initiate 1-2 hours after
thermals start with the likely origin in southern Wisconsin ahead
of the next upper level disturbance.


issued at 340 am CDT Wednesday Jul 30 2014

Water vapor imagery suggestive of upper level impulse diving southeast
through WI and appears to be reflection at surface in form of cool
front arcing from south of Green Bay WI to north of LaCrosse WI.
Isolated showers earlier over portions of northeast County Warning Area have
diminished with lingering middle cloudiness gradually thinning.
Surface ridge axis remains in place from northern plains through
the middle Mississippi Valley. Aloft... pattern unchanged and remains
highly amplified between deep long-wave trough over the eastern
Continental U.S. And western ridge... with local area continuing to reside in
northwest flow.


Short term...(today and tonight)
issued at 340 am CDT Wednesday Jul 30 2014

Persistence fcsting remains the way to go with stagnant northwest flow
pattern surface and aloft... and little change thermally (h85 temperatures
remain around 12c). Thus... today to look fairly similar to yesterday
with patchy middle clouds giving way to diurnal cumulus increasing in
coverage during afternoon. Expect to see isolated to low end scattered showers
and isolated storms mainly from around the quadrant cities North/East with aid
of WI impulse sliding southeastward. May have to watch for isolated stronger
cell with gusty winds and possibly small hail portions of
northeast Iowa and northwest Illinois with fairly similar Low Cape/high
shear environment to that of WI yesterday which saw of speckling of
severe reports mainly wind. 0-6km shear though prognosticated not to be
as strong as across WI yesterday (35-40 kts)... with latest runs of
rap and NAM suggesting 25-30 kts of 0-6km shear northeast of kdbq-
kvys line. This plenty strong enough /seasonally moderate/ to
support a few more robust and organized cells especially when
modified forecast soundings using temperatures in upper 70s and dewpoints in
middle/upper 50s yield sbcapes around 1000-1300+ j/kg. Nice inverted
v-type sounding with very steep low level lapse rates surface-700 mb
of 8.5-10c with lots of sub cloud dry air for evaporative cooling
to enhance downdrafts. Greatest potential for stronger convection
and best coverage looks to be just east/NE of County Warning Area with aid of entrance
region of 70+ knots 500 mb jet maximum and low level convergence along surface front
possibly lake enhanced.

Any convection to gradually diminish by late evening and early overnight
followed by mainly clear/partly cloudy skies and light winds. Could see
some patchy fog especially any areas that see rainfall. Lows will
be similar with coolest readings (low/middle 50s) in river valleys
and low lying areas... while elsewhere expect lows in the upper 50s
to around 60f.

Long term...(thursday through tuesday)
issued at 340 am CDT Wednesday Jul 30 2014

Below normal temperatures for much of this period with better
chances of rain next week.

Thursday through Saturday...highly amplified blocked pattern to
remain locked in place during this time frame. Deep trough in the
east will allow for short waves to dive southeast in the northwest
flow aloft. Will have chance probability of precipitation during this time with the highest
probability of precipitation in our northern and eastern County Warning Area. Not much change in the
temperatures with highs in the upper 70s/lower 80s and lows in the
upper 50s/lower 60s.

Sunday through Tuesday...the blocky pattern breaks down during this
time frame as a piece of strong energy from a deep closed low south
of the Aleutians pushes into the northern rockies...and flattens
the western ridge. This sets up a zonal flow briefly early next
week. This should trigger periodic thunderstorm complexes in or near
the dvn County Warning Area as a strong frontal boundary will meander over the
Midwest...on the edge of the heat dome centered in the southern
rockies/Southern Plains. For now will have chance probability of precipitation early next
week with the warmest day on Monday...with highs in the 80s.

Beyond this the European model (ecmwf) looks interesting developing a closed low and
tracking it across the Midwest later next week providing soaking
rains for some areas. This to be followed by another chilly airmass
dropping into the upper Midwest behind the system. Below normal
temperatures appear likely at least through the middle of August.


Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1233 PM CDT Wednesday Jul 30 2014

VFR conditions are expected through 06z/31. Isolated to widely
scattered convection will develop through 02z/31 which may result
in brief MVFR conditions from a shra/tsra. The probability of
convection affecting a taf site is under 10 percent. After 06z/31
mainly VFR conditions are expected. Light winds and slightly
higher moisture levels may result in some patchy ground fog
developing prior to sunrise.


Dvn watches/warnings/advisories...


short term...McClure
long term...haase

National Weather Service Glossary of Abbreviations