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Area forecast discussion 
National Weather Service Des Moines Iowa
339 am CDT Friday Jul 25 2014

Short term /today/...
issued at 338 am CDT Friday Jul 25 2014

Primary concern today will be convective trends and temperatures. Mesoscale convective system
expanded early this morning driven by both kinematic and
thermodynamic forcing. Positive differential vorticity advection
from short waves topping the southwestern Continental U.S. Ridge...tied into lobe from
southwestern Canadian upper low...continues from the eastern Dakotas into Iowa.
Strong 850 mb/700 mb Theta-E advection and 850 mb moisture transport ahead of
the short waves has also greatly fueled and influenced this elevated
convection. Main concerns appear to be frequent lightning and
locally heavy rains. MUCAPE axis continues to advance into Iowa with
system and three hour changes 500-1500 j/kg. Effective shear has
decreased peaking behind current mesoscale convective this and
combination of freezing levels exceeding 14kft has appeared to
diminish severe threat somewhat...hail in particular. This less
favorable severe environment has elevated the heavy rain threat
however with precipitable waters rising well through the 1-2 inch range and warm
cloud depths over 4km. Cells seem to be following southeastward upward
propagation vector. With mean wind normal to veering low level jet
this has keep the magnitude high enough to keep things somewhat
progressive. Do not expect much in the way of flash flooding with
recent dry spell...flash flood guidance continuing to rebound...and
mature crops.

Large scale forcing looks to wane into the afternoon so see no
reason to argue with hi res models ending precipitation by afternoon as mesoscale convective system
exits to the east. Attention will then turn to temperatures with warm
sector rapidly surging northward behind mesoscale convective system ahead of approaching surface
trough. Consensus of raw model temperatures would suggest maxes need to be
raised a tad so now have m80s/low 90s NE-SW. Resultant heat indices
are still in line with previous advisory however so no changes

Long term /tonight through Thursday/...
issued at 338 am CDT Friday Jul 25 2014

Main forcing exiting to the east and weak trough expected to pass
south of the forecast area by 12z Saturday. However...that being
said there will be only modest changes in airmass through Friday
night and Saturday with warm thickness and h850 temperatures still holding
across the area...especially the south half. Aloft...a rather weak
upper level wave at h500 will translate east into western Iowa by
midday Saturday. This will continue to slide southeast and induce
another weak surface wave over southern/southeast Iowa by late afternoon
and early evening Saturday. With instability remaining quite high
across the south Saturday afternoon and evening...weak lift and surface
convergence...some scattered thunder is expect for the late afternoon and
evening hours south third. Have expanded probability of precipitation a bit over the south.
Overnight Saturday night the thermal gradient at h850 increases
along with a renewed...but weak increase in h850 flow into the
boundary. This should result in a building of thunderstorms over the
south third during the middle to late evening hours as the low level
jet begins to show itself. Later in the evening the jet will
intensify and with the h500 wave moving east of the area...take most
of the convection out of the area after 06z. As has been the trend
for the past three changes to the timing and evolution of
the stronger cold front expected to drop south into the area on
Sunday. Tonights water vapor image shows the strong wave in Alberta
which is expected to move east and eventually phase with the h500
gyre over Hudson Bay... causing the entire eastern Continental U.S. Pattern to
amplify next week. The best forcing with the Canadian h500 wave
over the northern plains by Sunday will dig southeast across Minnesota
and Wisconsin with mainly diurnal showers across Minnesota/northern Iowa
dissipating by sunset as the colder air rushes into the region
Sunday afternoon with the passage of the first cold front.
Thus...rain chances look slim over most of the region Sunday with
only a slight chance for rain over the northeast. A second
backdoor front will March south during the night and exit the
southeast by 12z. Northeast flow will intensify during the
day...and this is when we will really appreciate the cooler
weather. H850 temperatures by late afternoon will hold to near 10c across
the area. This should lead to highs in the lower 70s northeast to
the middle 70s in the south. Clouds will likely build into the area
during the late morning and afternoon hours as well with the
colder air aloft. Cool high pressure will dominate the area
through Wednesday with only a modest warming aloft. Thus...expect
temperatures to hold pattern in the 70s to near 80 during the day and
in the 50s to lower 60s at night. Patchy fog may occur...
especially in low lying areas at times through the period. By
midweek a modest shortwave will drop south in the northwest flow
on the back side of the h500 low over the eastern Continental U.S.. this may
fire a few showers and isolated thunderstorms Wednesday afternoon...
though any instability remains quite low. Additional wave energy
tracking southeast may affect the area Thursday and Friday...
though confidence remains low toward the end of the week on both
timing and placement of any system dropping south with the
northwest flow. Cool weather will persist through the end of next


issued at 635 PM CDT Thursday Jul 24 2014

Patch of rain showers currently affecting area may produce a brief local
MVFR ceiling/visibility in the next couple of hours before dissipating...but
probability at a given terminal is low. Thereafter expect a break
in precipitation of several hours with a return to south-southeast winds and high
ceilings...but overnight after 06z anticipate more thunderstorms and rain development
though coverage is questionable so have handled with just thunderstorms in the vicinity for
now. By 06z issuance convective trends may be revealing themselves
allowing for more detail in the tafs. By midday Friday expect
storms will have ended with VFR conditions prevailing during the


Dmx watches/warnings/advisories...
heat advisory from noon today to 7 PM CDT this evening for Adams-



Short term...small
long term...rev

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