Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
735 am CDT Friday may 29 2015

issued at 336 am CDT Friday may 29 2015

As of 3 am CDT...latest GOES WV loop showed a trough over
the western U.S. With a middle-level center of circulation over
Wyoming. A dry slot was punching through Utah and Colorado on the
southern flank of the middle-level low...while broad northward WV
transport was evident downstream across the plains and Midwest.

Locally...the intensity of the isolated/scattered showers has
increased over the past hour mainly in a zone about a County wide
from Iowa City up to Dubuque with reflectivity values as high as 50-
55 dbz. lightning strikes have been detected. Surface
temperatures were mild in the middle 60s.


Short term...(today and tonight)
issued at 336 am CDT Friday may 29 2015

Today...southwest flow at 700mb and weak warm air advection combined with
increasing conditional instability will result in chances for
isolated to scattered showers and thunderstorms through the
afternoon. Recent model guidance and synoptic/mesoscale analysis
favors isolated coverage this afternoon as there will not be a surface
front in the area or forcing for large-scale ascent.

The atmosphere will be anomalously moist so any showers or storms
that form today or tonight have the potential to produce heavy
rainfall rates approaching or exceeding 1 inch/hr. 850mb height
fields expose a very moist trajectory with easterly winds across
the Gulf of Mexico veering to the south-southwest over the
southern U.S. Into the upper Mississippi Valley.

Later this evening into tonight...a positively tilted shortwave
trough will move into the western portion of the forecast area but
begin to shear out with time. Low-level convergence along the
front and middle to upper level ascent tied to positive vorticity advection and ageostrophic
divergence in a right jet entrance region will act to increase
shower and storm coverage most likely after 7 or 8 PM this evening
and prior to 4 am Saturday. Thus have highest probability of precipitation of 70-80
percent during the evening and the first half of tonight.
Kinematics do not support severe storms but with moderate levels
of instability developing through the day as MUCAPE peaks near
1500-2000 j/kg a few strong or marginally severe storms are

Long term...(saturday through thursday)
issued at 336 am CDT Friday may 29 2015

High pressure following the early Saturday cold frontal passage will
provide cool and dry conditions for much of the weekend into early
next week as it migrates from the northern plains to the Great
Lakes. Temperatures and humidity will then edge back upwards from
middle to late week as the upper level pattern returns to SW
flow...allowing Gulf moisture to rebound northward.

Saturday...slightly slower 00z model solutions showing the upper
level trough passing around midday or early afternoon will require
keeping at least chances for showers going through the morning.
Limited instability should eliminate risk of thunderstorms in the
morning...but based on the slower timing that may tap into some
afternoon instability...will carry a slight chance for thunderstorms
in the far east and southeast. Cold air advection on brisk north-NE
winds...along with cloud cover in the weakly cyclonic low level flow
will limit highs to the 60s.

Drier air and middle level subsidence should win out Saturday night to
lead to partly cloudy skies while a sufficient pressure gradient
keeps northeast winds going at least from 5 to 15 miles per hour through the
night. This should hold mins to mainly the 40s...whereas some upper
30s would be possible if decoupling suggested by the
WRF. Sunday looks mostly sunny with east winds out of the Great
Lakes high again holding highs in the 60s.

Monday through Thursday will be progressively warmer and
humid...especially Wednesday as beyond the mostly zonal upper flow
becomes more amplified and southwesterly over the region. Highs rise from
the 70s Monday and Tuesday...into the 80s Wednesday and Thursday with lows only in the
60s by late week as dewpoints creep upwards on the low level return
flow. While the forecast has low chances for showers and
thunderstorms from Wednesday night Onward...the better chances will hold
off until Friday or possibly the weekend at the earliest...when the
developing frontal system and upper trough in the plains is shown
advancing eastward.


Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 727 am CDT Friday may 29 2015

Isolated showers and thunderstorms will be possible through
the afternoon. Storm coverage should increase this evening as a
shortwave trough approaches from the west.

Storms are most likely at kcid and kdbq where best forcing and
instability is forecast to develop so mentioned thunderstorms and rain during the
late evening/early nighttime hours at these terminals. Aside from
the storm potential ceilings will slowly fall through this taf period
reaching MVFR...and possibly IFR after 06z/Saturday. Westerly
winds will increase early Saturday morning behind a cold front.


Dvn watches/warnings/advisories...


short term...uttech
long term...sheets

National Weather Service Glossary of Abbreviations