Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
1238 PM CDT sun Aug 2 2015

issued at 1237 PM CDT sun Aug 2 2015

Elevated convection has continued with isolated showers and brief
thunderstorms due to the steep middle level lapse rates and moisture
advection. Recent satellite imagery showed this focused on an
apparent elevated boundary oriented SW to NE from about Ottumwa to
Galena. At the surface...winds have become gusty as expected from
the southwest while temperatures as of noon were well into the
lower 80s. Despite the deeper mixing under partly to mostly sunny
skies...dewpoints were climbing into the lower 70s over most of
the area.

At noon...the cold front was roughly from west central WI SW to near
kfsd. 12z models and high res convective models are in general
agreement with strong to possibly severe storms holding off until
very late afternoon or evening along the cold front prognosticated
roughly from dbq to dsm at 00z. Deeper shear developing during
this time...with high convective available potential energy likely in place will support severe
storms and the Storm Prediction Center 16z update has maintained a slight risk over
most of the forecast area...with enhanced touching northeast
Stephenson County.

Will be updating the forecast to focus probability of precipitation close to the
late afternoon frontal timing with little change to highs that
will push the upper 80s to near 90.


issued at 329 am CDT sun Aug 2 2015

An agitated altocumulus castellanus deck associated with a quick moving vorticity maximum in
the steering flow was located from Escanaba Michigan to Hastings NE.
Extremely isolated convection has formed in this band as it
marches to the southeast at around 20 kts. At the surface...dewpoints are
pooling across the area and relatively clear skies remain across
the area. With the pooling moisture....temps have been slow to
drop across the area. Guidance suggests an 850 mb thetae push of
near 50 degrees into the area near 12z. This and the vorticity maximum will
dominate our weather in the next 6 to 12 hours.


Short term...(today and tonight)
issued at 329 am CDT sun Aug 2 2015

Main forecast concern for the short are the chances for
thunderstorms and rain today. There are two chances with the first
one either ongoing at 12z or starting shortly thereafter. The
other is associated with a cold front that will move through the
area today and into tonight. Some of the storms later this
afternoon and evening could be strong to severe.

This morning the altocumulus castellanus deck moving into the area will encounter an
850 mb thetae push. These two are forecast to be coincident across
the County Warning Area near 12z. This could lead to the development of showers
and thunderstorms across the area. Large scale models hint at this
possibly happening...however the mesoscale nature of the forcings
are difficult for the models to handle correctly. Perusing the
cams...which do better with mesoscale is clear that
these models do suggest additional thunderstorm development this
morning with the wave. At this time...these storms are not
expected to be severe. A strong storm with small hail could be
possible. I do not have high confidence in this occurring so
decided to go with schc probability of precipitation until the altocumulus castellanus becomes better
agitated. Any storms this am could leave outflow boundaries that
may influence convection later.

This afternoon...guidance suggests that mixing will result in high
temperatures in the 90s across the area. Soundings indicate the eml has
resulted in drier air aloft. As this drier air is mixed down...the
temperatures should increase and dewpoints should lower. Used mosguide
for the dewpoints as they seemed to have a better handle on what
was going on now and mixing the dewpoints lower this PM. As a
result...heat indices will remain below criteria.

Late this afternoon a cold front will move through the area.
Looking at the thunderstorm potential...there will be sufficient
deep layer shear for isolated cells. With the main forcing to the
north...these storms may be slower to grow upscale into a mesoscale convective system.
Lapse rates in the middle levels suggest that hail will be the main
threat. With the high temperatures and likely higher storm
bases...damaging winds could also be possible. Low level shear
and hodos are indicative of the possibility of tornadoes as well.
In fact they are very favorable for possible tornadoes.
However...the thermodynamic environment is not. I think the threat
of tornadoes is higher with an mesoscale convective system instead of a supercell storm
that may be the first Mode of development. Regardless there is a
threat for severe weather from these storms.

Looking at the cams...the NAM 4km has the bulk of the storms and
bowing structures across WI and east Illinois. This model probability of precipitation short lived
isolated cells that struggle to grow upscale into a mesoscale convective system
structures. The wrfnmm and arw are eerily similar in which a
frontal mesoscale convective system comes through our area instead of to the east.
Regardless these models produce storms and thus give ME higher
confidence that convection will occur. Did not go higher than chance
as I do not have an idea of which of the solutions will occur.
Later shifts may be able to increase probability of precipitation across the area as the
day progresses and a clearer solution is seen.

Long term...(monday through saturday)
issued at 329 am CDT sun Aug 2 2015

Monday and Monday night... northwest flow aloft will dominate early in the
week as anomalously deep upper low (500 mb heights of 550-555 decameters
equating to roughly 2+ Standard deviations below climatology per naefs) remains
parked near Quebec. This flow pattern coupled with high pressure ridge
building in will result in cooler (near normal) and drier conditions
to start the week. A surface front is anticipated to stall out from the
Central Plains to the Appalachians by Monday evening. Isentropic lift and
weak low amplitude shortwaves will provide focus for precipitation chances
near and on cool side of boundary...with the far southern County Warning Area potentially in
close proximity to some of this activity.

Tuesday and Tuesday night... overall pattern not showing much change
during this period with upper low slow moving through Ontario... and
County Warning Area staying on cool side of stalled frontal zone from the plains through the
middle-Mississippi Valley. Thus... more of the same with pleasant conditions
with near to slightly below normal temperatures and comfortable humidity levels...
and any rain chances mainly to our South/West in vicinity of stalled front
with southern County Warning Area standing any chances for rain being in closer proximity to

Wednesday through Saturday... continued gradual drawn down on temperatures to below
normal expected into late week with influx of cooler air (850 mb temperatures
lowering into range of 12-16c) in wake of departing Ontario upper low.
In addition...likelihood of intervals of cloudiness with series of upper level
disturbances propagating through region in the semi-zonal to northwest flow
to aid in keeping temperatures below normal. With surface front nearby will maintain
rain chances through Thursday then trends support drying out late week into the
weekend with another bout of Great Lakes ridging and influx of cooler...drier air.


Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1237 PM CDT sun Aug 2 2015

Isolated...high based showers may affect the mli terminal early
this afternoon. Otherwise...VFR conditions will be in place with
gusty southwest winds and warm temperatures. Scattered
thunderstorms are expected along a cold front dropping south into
the area this evening and have focused tempo groups for this
timing at Cid...dbq and mli. Confidence in timing and coverage is
lower for brl...where a prob30 group was utilized.
Overnight through Monday morning...light north-northwest winds are expected
with mostly clear skies.


Dvn watches/warnings/advisories...


short term...Gibbs
long term...05

National Weather Service Glossary of Abbreviations