Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Sioux Falls South Dakota
538 PM CST Sat Dec 20 2014
Short term...(this evening through sunday)
issued at 221 PM CST Sat Dec 20 2014
Much of the same expected tonight into tomorrow with the low
clouds hanging tough over a majority of the area. Do not really
expect the clearing line to move much through tomorrow...with only
Gregory...Brule and portions of Charles Mix counties seeing any
clearing and sun. Signal that the denser fog may encompass more of
the area tonight...making it to around the Interstate 29 corridor.
So will have to keep an eye on this...for now increased mention of
fog in the grids. GFS and rap suggest low level moisture depth will
increase tonight ahead of a weak upper wave that crosses the region.
This happens mainly across portions of northwest Iowa from Sioux
City to Spencer. So could potentially see some patchy drizzle or
freezing drizzle in that area...but anything should be light.
Given the cloud cover...diurnal range of temperatures will be small.
Will not fall much tonight...with upper 20s to low 30s expected.
Then on Sunday we will only warm up several degrees...with 30s
across most of the region. Will start to become a bit breezy out of
the south by later Sunday...with gusts around 25 miles per hour east of the
Long term...(sunday night through saturday)
issued at 221 PM CST Sat Dec 20 2014
Precipitation likely to develop on Sunday...with the better chances
in the eastern County Warning Area. While the main warm front is to the north of the
area...the pv anomoly and cold front aloft swing through Sunday
night and should allow what will be mainly rain. The northern and
western County Warning Area could see some mixed precipitation but the chances for
precipitation a little lower in these locations so hopefully little
or no accumulation. Temperatures remain pretty mild through the
night as well with low pressure wrapping up to the north and west of
the area so mainly southerly or westerly flow. Lows generally from
the lower to middle 30s.
Looks like everyone should warm to at least the middle 30s on Monday
with lower 40s likely in parts of northwest Iowa. The daytime
chances for precipitation should not be real high and will keep most
locations in the chance category as the area waits for the wrap
around to set up late Monday afternoon into Monday night. Right now
looking like mainly a rain to snow transition with little chance for
sleet and freezing rain Monday and Monday night.
The chance for snow will linger into Tuesday but amounts will remain
fairly minimal. When all is said and done looking at about a half an
inch to an inch over northwest Iowa to 2 to 3 inches from Huron to
Marshall. A wave tracks through to the north of the area on Tuesday
which will bring some drying aloft and bring an end to the threat
for precipitation Tuesday night into Wednesday.
The upper level pattern remains fairly active through the extended
period with one wave exiting on Wednesday and another moving into
the area Friday into Saturday. Still some decent differences for the
incoming system so not overly confident on what to expect but right
now looks cold enough for all snow...just a matter of whether or not
this is a system that moves through rather quickly or one that takes
on just a touch of negative tilt like the European model (ecmwf) suggests and brings
in a better chance for heavier amounts. Unfortunately no way to
really figure this out right now so will continue to carry a chance
of snow Thursday night into Saturday. For now Wednesday and Thursday
look mainly dry with slightly below normal high temperatures through
the period. Overnight lows will likely be at or above normal during
Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 538 PM CST Sat Dec 20 2014
Poor aviation conditions will continue through the period...with
IFR to LIFR ceilings and visible likely. Not really expecting much change
from the current conditions...so mainly went with a persistence
forecast. Some signs that denser fog and LIFR ceilings will spread a bit
further east tonight toward Interstate 29 and into higher elevations
of southwest Minnesota tonight. Stronger boundary layer winds develop by
Sunday afternoon...which could help lift ceilings into MVFR range
for a few hours toward the end of the taf period.