Scientific Forecaster Discussion
Area forecast discussion
National Weather Service quadrant cities Iowa Illinois
1157 PM CDT Sat may 23 2015
issued at 338 PM CDT Sat may 23 2015
Mostly sunny...warm and dry conditions prevailed this afternoon over
the forecast area. 18z surface analysis indicated the area was on
the far western periphery of surface high pressure over middle Atlantic
coast...but closer to the upper level ridge axis that stretched from
the southeast into the western Great Lakes. Resulting subsidence and
dry air was keeping thicker cloud cover at Bay to the west...while
area radars continued to show high based showers of mainly virga
continuing to approach and then fade over our western and SW
counties. Much more active weather was looming not far to the
southwest...where radar mosaic showed a linear mesoscale convective system slowly moving
across Kansas...OK into northwest Texas. This was where a middle level shortwave
lifting around the based of the upper low over Utah was interacting
with feed of Gulf moisture...from the Texas Bend region into SW Kansas.
Short term...(this evening through sunday)
issued at 338 PM CDT Sat may 23 2015
Forecast remains centered on the onset of more active weather...that
has the potential to provide wet and potentially stormy periods over
the remainder of the Holiday weekend. The initial question is how
much the ongoing mesoscale convective system over the plains affects convective evolution
tonight. The shortwave/vorticity maximum in the SW flow over the Central
Plains is shown by all models lifting northeast into Iowa tonight. The
position of the 850 mb jet and strongest moisture convergence
initially focuses into western and central Iowa late tonight...but
then veers some...depending on the model...to aim into eastern Iowa
and northern Illinois...especially from 12z to 18z Sunday. Convective
high res arw and nmm models would keep the developing showers well
to the west of the forecast area tonight with the initial
impulse...then suggest convective development over the forecast
area Sunday afternoon as a subtle surface warm front...and focus
for low level convergence...lifts north through the region.
However...based on the poor handling of the on-going complex by
these models in the plains...there is low confidence in these
Based on current trends and the likelihood of the on-going plains
system being the dominant feature in the near term...maintain a dry
forecast through this evening with only sprinkles possible from the
virga moving into the northwest early. Brought in low probability of precipitation into the far SW
around midnight...spreading northeast and increasing to cover most
of the area by morning with likely probability of precipitation confined to the west. There
is a high likelihood that northwest Illinois will remain dry through the night.
With meager middle level instability...have left out mention of
Sunday...the combination of an increasing low level moisture
feed...weak warm front lifting north...building middle level
instability and passage of one or more middle level shortwaves will
require at least likely probability of precipitation most of the forecast area.
However...forecast confidence remains low. Significant portions of
the day may end up dry...especially if the more convective looking
high res models verify and also if the plains mesoscale convective system continues to
interrupt the moisture feed and low level jet as it continues east-southeast
through MO and Arkansas. For now...will go closer to the more pessimistic
models with mostly cloudy skies which would limit highs to lower 70s
north to middle and upper 70s south. The limited insolation would keep
surface based convective available potential energy and MUCAPE low enough to limit thunderstorms to
isolated through the day...but then potentially more scattered by
late afternoon with the support of deeper shear and arriving steeper
middle level lapse rates. At this time...neither appear strong
enough to produce severe storms and quantitative precipitation forecast during this period is from
around .10 east to a .25 to .50 range west.
Long term...(sunday night through saturday)
issued at 338 PM CDT Sat may 23 2015
Sunday night...scattered showers and thunderstorms are likely but
widespread heavy rainfall is not anticipated.
As low pressure moves from western Iowa to south-central
Minnesota...southerly surface winds will keep the forecast area
relatively mild and humid through the night. Forecast precipitable water values
near 1.75 inches are approximately 2.5-3 Standard deviations above
the mean or the 99th percentile for the end of may.
Instability...700-500mb temperature lapse rates are only forecast to
be near 5.5 c/km and MUCAPE value will fall from near 500-1000 j/kg
around 7 PM Sunday to less than 500 j/kg after midnight Sun night.
Forcing...light upper level winds and even some ridging at 300mb
translates into little to no large scale forcing for ascent.
However...a shortwave rotating around a parent middle-level low over
Wyoming is forecast to move across the forecast area after midnight
Sun night and 700mb warm air advection will precede this feature as 850-700mb
southerly winds increase.
Monday...highs in the lower 80s and dewpoints in the 60s will yield
moderate levels of conditional instability as MUCAPE values increase
to 1500-2000 j/kg or greater. However..E Iowa/west Illinois will be
between two shortwave features....so expect mainly dry weather
through the day. Then...one of the middle-level shortwaves will slide
up from the southwest and cross the dvn County Warning Area during the late evening
and early nighttime bringing a slightly better chance for scattered
showers and storms.
Tuesday...the main upper low that was over Wyoming on Sunday night
and Monday will finally kick out of the central rockies...merge with
a vorticity maximum to its south...and then propagate through the upper
Midwest. This will be a weakening feature which will again produce
non-severe scattered showers and storms. Forecast highs are in the
middle to upper 70s.
Wednesday and Thursday...southwest middle-level winds will persist
through this period as a low amplitude trough remains over the
central and northern rockies. Daytime instability and subtle
embedded vorticity. Maxima warrant 20-30 probability of precipitation. Anticipate above average temperatures
in the upper 70s to lower 80s. The lack of a low-level frontal
boundary and weak 0-6 km bulk shear will put a limit on storm
coverage and severity.
Friday...models are hinting at slightly warmer temperatures into the lower
to middle 80s with increasingly humid conditions as dewpoints
potentially reach the middle to upper 60s. An approaching cold front
from the north and northwest may result in scattered showers and
thunderstorms with greater coverage than the past few days.
Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1153 PM CDT Sat may 23 2015
06z tafs updated to take a Stab at refining most likely periods of
rain. The initial rain during the early morning will be most
persistent at kbrl. Kbrl/kcid/kdbq are also favored for a period
of showers and isolated thunder through late Sunday morning. There
may be a brief lull in the precipitation before additional development
from late afternoon on.