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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
1205 am CDT Friday may 29 2015

issued at 1204 am CDT Friday may 29 2015

850 mb moisture transport continues to spread higher precipitable
waters northeast farther into the dvn County Warning Area...and ranged from near 1
inch in northwest Illinois to 1.4 inches in our southern and western County Warning Area.
Instability is minimal and there is no significant short wave to
focus organized convection. However...Doppler radar does show a
few showers in our western County Warning Area and farther south into central MO
but much of this is falling out of a middle deck. The hrrr indicates
scattered showers and isolated thunderstorms the remainder of
tonight with a slight increase in speed of the low level jet. I
have updated the grids to reflect this latest thinking.

Update issued at 705 PM CDT Thursday may 28 2015

850 mb moisture transport continues to bring higher precipitable
water (1.6 inches) into our southern and western counties early
this evening. Much drier air is noted in northwest Illinois where precipitable waters are
less than an inch. Showers and a few thunderstorms were surging
northward especially in our far western County Warning Area. However...deep layer
shear is weak and sbcapes are less than 500 j/kg so not expecting
severe weather...but the stronger storms will produce locally
heavy rainfall.

As moisture transport and forcing increase showers and a few
thunderstorms will continue to develop/spread over all of the County Warning Area


issued at 311 PM CDT Thursday may 28 2015

12z upper air analysis had an 850mb high over the Great Lakes and Ohio
Valley. A weak low level jet ran from Texas into eastern Kansas that was
helping to support nocturnal convection along the Kansas/MO border.
Satellite trends through middle afternoon has weak convective complexes
from western Iowa into Missouri. Radar has mainly showers across
Iowa with more thunderstorms than showers across Missouri.

18z surface data has high pressure over the Great Lakes with an implied
front/boundary from the Ohio Valley into central Missouri. Dew
points were in the 50s from the Great Lakes into the northern plains
with 60s from the Central Plains into the Ohio Valley on south.


Short term...(this evening through friday)
issued at 311 PM CDT Thursday may 28 2015

Based on radar/satellite trends convection will continue to develop
across Missouri through late afternoon with some isolated to
possibly scattered showers moving into the far south and southwest.
The late arrival of the convection into the area suggests it should
be in a decaying phase by early evening.

This evening the better support aloft starts moving out of the
plains so isolated to scattered showers should become a bit more
numerous across the western third to half of the area through
midnight along with some thunderstorms.

After midnight the better forcing/support starts moving through the
area. The better coverage of showers and some thunderstorms will be
over the northwest third of the area but again should only be
scattered in nature. East of the Mississippi isolated to low end
scattered showers and a few thunderstorms should develop after

Overall rainfall tonight should generally be a tenth of an inch but
some amounts approaching a quarter inch might be seen in the
northwest third of the area.

Scattered showers and a few thunderstorms should be seen across the
area Friday morning as the better support exits the area. The late
morning and early afternoon hours may see very little in the way of
convection due to the lack of stronger forcing aloft.

By late Friday afternoon...the main front moves closer to the area
and the overall forcing aloft increases. Thus convection will be on
the increase during the peak heating hours.

Long term...(friday night through thursday)
issued at 311 PM CDT Thursday may 28 2015

The cold frontal passage will occur late Friday night through early
Saturday morning. While precipitable water values well above normal are
forecast...along with cape sufficient for strong thunderstorms...we
will be lacking a strong forcing mechanism...with no definite short
wave passing through the Midwest at the time of the frontal passage. I am
worried that with ample cape...convective trends will form small
clusters and eat into the higher cape values farther south...and
with stronger middle level winds to the north...other widespread
activity may track over northeast Iowa to Wisconsin. We have been
advertising categorical probability of precipitation...but given this uncertainty on
coverage...I am electing to lower probability of precipitation to likely for Friday night.
Isolated amounts over 1 inch are likely...but widespread amounts
otherwise should be a half inch or less.

Early Saturday...any remaining showers will exit the area. Cooler
air will arrive Saturday morning...and highs will likely not climb
more than 10 degrees above morning lows in many spots. This
comfortable air should last through much of the extended...with
highs remaining below normal through Monday. By Tuesday...with low
pressure developing in the plains...and a very weak upper low cut
off over the southeast slipping southeast...return flow should
establish itself. Milder air...and eventually above normal
temperatures and typical June humidity...will arrive by middle to late


Aviation...(for the 06z tafs through 06z Friday night)
issued at 1205 am CDT Friday may 29 2015

Mainly prevailing VFR conds through this taf cycle. Scattered showers
and isolated thunderstorms are expected overnight and into Friday
morning. There should be a break in the rain later Friday morning
but then thunderstorms will increase again Friday afternoon into
Friday night as a cold front approaches from the northwest.


Dvn watches/warnings/advisories...


short term...08
long term...Ervin

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