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Area forecast discussion 
National Weather Service Des Moines Iowa
1236 PM CDT Thursday Oct 2 2014

Short term /today/...
issued at 400 am CDT Thursday Oct 2 2014

Dense fog that socked in much of the County Warning Area tonight will continue
through the middle morning hours...though there is some uncertainty as
to how long it will linger. The duration of the fog will likely be
directly tied to the arrival of precipitation this morning with the low
level col and moisture pooling only gradually shifting eastward by
18z. Thus...fog will likely end from west to east through the morning
and have left the 14z expiration time of the dense fog advisory
alone for the time being.

The bigger story is the potent upper level shortwave over the
western High Plains rotating around the mean western Continental U.S. Trough
today. Showers and thunderstorms ongoing over Nebraska at the nose
of the upper level jet are expected to reach the western County Warning Area by 12-
13z this morning...with several rounds of showers and storms
possible throughout the day out ahead of the wave. The best threat
for severe weather appears confined to the southeast County Warning Area. Overnight
convection over Missouri shunted the mean warm front further south
than is currently prognosticated by the models. How far north the boundary
recovers this afternoon will determine the northern extent of the
severe potential with a Stout 700-800 mb cap ahead of the front.
NAM/RUC soundings for suggest surface based storms may be
possible near the Iowa-MO border and with 0-1 km shear values around
20 kts...a tornado cannot be ruled out. Convective initiation
appears to be in the 18-21z timeframe...with the storms rapidly
tracking to the east northeast. The main synoptic cold front
begins sweeping from west to east over the County Warning Area after
18z...heralding the arrival of colder air for the extended.

Long term /tonight through Wednesday/...
issued at 400 am CDT Thursday Oct 2 2014

Pattern migration continues through the period as upper level system
will amplify quickly in the next 36 hours. Tonight...subjective h850
analysis shows first 2 frontal waves that have passed through the
region so now entering the upper Great Lakes and todays
just exiting the forecast area. The first cold front at h850 is now
along the Dakotas Minnesota border SW to North Platte NE where it
intersects a developing Lee side closed low over the Panhandle of
OK/TX. The Arctic boundary associated with a stronger northern
stream short wave over Canada is well back to the northwest...just
passing through Glasgow Montana at 00z Thursday. The other major features
continue to be the anchor high over New England and an unusually
strong subtropical high just east of Florida. An impressive ribbon
of 15c dew points stretches from southern Texas to the NE/KS/IA
border...just ahead of the SW upper level system. At h500 the main
and last SW wave is currently digging into the base of the trough
..back in the SW US Four Corners area. By 00z this evening the h500
wave will just be recurving northeast over eastern OK/KS...a bit
farther south and slower than previous days forecasts. Both the Gem/Euro
now have a more broad...elongated surface trough passing through the
region today and departing by 06z this evening as the first h850
cold front moves into central Iowa by 12z Friday. Sufficient
subsidence across the region should allow for some breaks in cloud
cover by sunrise. As the cold air invades the region tomorrow will
see steepening lapse rates during the afternoon/evening hours. At
this appears that h850 winds will support surface gusts from
40 to nearly 45 with the higher gusts over the northeast quarter of
the forecast area on Friday...remaining near to just below Wind
Advisory criteria.

As the southern and northern streams phase...the models continue to
have slightly differing solutions on the timing and placement and
evolution of the deepening phased system. These differences yield
some modest uncertainties on the forecast. The 12z Euro is a bit
slower in phasing the streams compared to the 12z Gem. With that
being said...short wave arriving Friday night still looks to clip
northeast Iowa as it wobbles/spins over Iowa/MN/WI between 06z and 15z
Saturday with some precipitation northeast. Bufr soundings...thickness...
and ice introduction between 06z and 12z all point to potential for
rain/snow mixture over the far northeast and have added it to the
forecast. Besides that...soundings again turn adiabatic as a second
cold pool forms aloft associated with cold core h500 lows and this
feature quickly steepens lapse rates again overnight. This should
lead to a few hour period of higher wind gusts from 06z to 12z
Saturday morning over the northeast from kmcw to Kalo especially.
Winds may approach 45 miles per hour or higher for a brief period of time.
With the winds being brisk most of Friday night...frost is basically
out of the question. However...freezing temperatures are still
possible and with a well mixed boundary layer at times...and
generally uniformly strong cold air advection Friday and Friday
night...much of the area may be near or just above the freezing mark
by Saturday morning. Temperatures guidance shows a hard freeze
across the north quarter by 12z Saturday...but winds may prevent the
cold air from settling out. Will need a bit more time to Iron out
the details of Friday nights lows. Wind chills early Saturday
morning range from about 20f north to 30f in the south as well.

Concerning the big picture...the overall similarity in the overall
h500 evolution over the northern hemisphere in the next 96 hours is
similar for the GFS/Gem and Euro. All solutions point toward a
rather cold period from Friday through early next week...a bit
longer than earlier anticipated as a large hemispheric anchor low
deepens over eastern Canada near Hudson Bay. This keeps the region
in rather strong northwest flow through the remainder of the period. There
is yet another short wave that arrives either Sunday night or early
Monday...which briefly brings showers and reinforces cold air into
the area for another day or two. Finally by Tuesday...the upper
level h500 flow becomes more zonal and promotes a period of rapid
warming through Thursday of next week. Highs should recover into the
60s to lower 70s by Tuesday and well into the 70s on Wednesday.
Similarly...lows will rise from the 30s to middle 40s Monday night to
the 40s and 50s by Wednesday. Both thickness rises and h850 warm air
advection strongly support this quick transition back to milder


issued at 1232 PM CDT Thursday Oct 2 2014

Frontal boundary near low pressure system will keep MVFR and IFR
conditions at well as widespread rain showers/thunderstorms and rain through much
of the period. Visibilities and ceilings may occasionally drop to
LIFR early in the period...though will primarily remain above.
System will push east...with ceilings lifting to low VFR near
06z. However...north to northwesterly winds will increase and will
be very strong and gusty from 12z through end of period.


Dmx watches/warnings/advisories...



Short term...skow
long term...rev

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