Area forecast discussion 
National Weather Service Denver/Boulder Colorado 
251 PM MDT Sat Apr 25 2015 

Short term...(this evening through sunday) 
issued at 251 PM MDT Sat Apr 25 2015 

Upper trough currently digging into the Great Basin with southwest 
flow increasing over Colorado. Weak shortwave trough ahead of it 
has sparked convection off over Colorado. A surface low over 
south central Colorado will push further south as high pressure 
from the Great Plains pushes in...advecting in higher dewpoints. 
Increasing qg lift and convective available potential energy of 500-750 j/kg out on the northeast 
plains will allow stronger storms mainly east of an eastern Weld 
County to northern Lincoln County line. Not expecting too much in 
the way of severe strength storms as winds aloft are not that 
strong and given a more saturated airmass. Could still see gusts 
to 40 miles per hour and small hail. Convective activity further west toward 
the urban corridor will begin drier this afternoon with gusty 
winds and virga...then as higher dewpoints filter in...there will 
be a better chance for rain. Cape should stick around over the 
northeastern plains most of the night with increasing 
moisture...will likely see showers/storms continue through the 
night. Mountains have snow levels around 10kft this afternoon and 
it will likely only lower to about 9kft tonight. Showers and 
thunderstorms will continue through the evening with some areas 
able to pick up a few inches but it will be spotty in nature. 

As flow turns more easterly and more moisture is advected in 
overnight...will likely see low stratus and fog develop over the 
plains especially as it hits up against the higher terrain. Flow 
will turn more southeasterly which will likely pile moisture 
against the northern foothills and Cheyenne Ridge. Have 1/2 mile 
visible expected overnight...may need to think about a dense fog 
advisory over this area. 

Upslope flow will increase in speed and depth tomorrow as upward 
qg motion remains overhead as the trough deepens into a cut off 
low over northern New Mexico. Latest GFS and ec model runs have 
trended slightly cooler. Therefore have issued a Winter Storm 
Watch for the Front Range mountains starting noon tomorrow 
with the east slopes the main focus. There will be plenty of will depend on temperatures on how much snow 
accumulates. Also...some cape being forecasted over the high 
terrain which could increase amounts in localized areas. For 
now...expect 2 to 8 inches to fall over elevations above 9000 feet 
tomorrow. Over the plains...upslope flow will keep 
clouds....showers and fog around with a slight chance of 
thunderstorms. This will limit heating during the 
temperatures are expected to stay in the upper 40s to middle 50s. 
Easterly surface winds will increase through the day. 

Long term...(sunday night through saturday) 
issued at 251 PM MDT Sat Apr 25 2015 

Upper level low will track slowly east across northern/central New 
Mexico Sunday night. Very moist upslope flow with around 0.6 inch 
precipitable water will remain over the forecast area through the 
night. The heaviest precipitation will likely continue from the 
Sunday afternoon hours into Sunday evening in/near the Front Range 
when upslope component is deep and strongest near 20 knots. It 
should be noted the latest 12z model trends have turned a bit 
colder...allowing for the snow level to drop quicker in the 
evening during the heaviest precipitation. Snow level could now lower to 
around 6500 feet by early Monday morning per latest data. 

We ran the local orographic precipitation model and some rather 
large numbers were kicked out for the Front Range mountains in 
both quantitative precipitation forecast and snow...especially Sunday evening. There will likely 
be a large variation over the higher foothills and Front Range 
mountains with total snowfall amounts literally ranging from just 
an inch or two to upwards of 20 inches. Overall forecast reflects 
a blending of the orographic model with synoptic/mesoscale model 
consensus. The Winter Storm Watch for the Front Range mountains 
above 9000 feet will remain in effect for Sunday night until noon 
Monday...although snow intensity is expected to gradually decrease 
as the upslope component and q-g support both weaken through early 
Monday morning. There is some potential a watch or jump to a 
warning could be needed in the higher foothills...but given only 
the latest model run has trended colder will allow further 
diagnosis before adding any appropriate highlights there. Most 
likely...the heaviest snow in the foothills would be confined to 
elevations above 8000 feet since the heaviest precipitation occurs early 
in the period when it is warmer...but will continue to monitor. We 
will likely see the snow level eventually drop down to the Palmer 
Divide late Sunday night into Monday morning with at least a 
couple of inches there. Still expect nothing but rain from the 
Denver metropolitan area north and east across the plains. The bulk of 
the heavier precipitation will be shifting quickly tomorrow evening to 
the Front Range...but a few showers should also continue on the 
eastern plains into Monday morning. 

By Monday afternoon...system support continues to drop away from 
the region so we will see a gradual decrease in precipitation 
into Monday evening. If skies clear Monday night then could see 
below freezing temperatures even on the plains...but another short wave 
could keep temperatures a bit warmer with some clouds. That weak wave may 
attempt to produce isolated showers Tuesday...but models are 
trending away from this scenario for now. 

Then warmer and drier conditions expected for Wednesday. Above 
normal temperatures will likely hold until next weekend when 
a glancing short wave is possible. We do anticipate moisture to 
gradually build under the ridge with isolated to scattered 
afternoon and evening convection in the mountains by 
Thursday...and over all of the forecast area Friday and Saturday. 


Aviation...(for the tafs through 00z Sunday late afternoon) 
issued at 251 PM MDT Sat Apr 25 2015 

Expect showers and thunderstorms moving off the higher terrain 
and across the Denver area through about 00z...then high res 
models continue showing a break in activity through 03z...then 
another wave of activity through 06z. Main threat will be gusty 
outflow winds up to 40 knots. Ceilings will briefly fall below 
6000 feet under the stronger showers and storms. Easterly winds 
will transport low level moisture into the area late 
tonight...after 06z Sunday. This will bring low clouds and areas 
of fog. By 12z Sunday...expect ceilings will be below 1000 feet 
with a good chance of fog with visibility a half mile or lower. 
Hard to forecast right now if it will be drizzle or rain showers 
overnight. Rain is expected to increase Sunday with continued IFR/LIFR 

Wind direction continues to be very tricky as a Denver cyclone 
is beginning to be affected by convection. Overall expect winds 
to generally trend toward a northerly direction this afternoon as 
the main surface low heads south...then easterly later this 
evening...veering to southeasterly late tonight into Sunday. These 
southeast/east winds will increase in speed through the day 


Bou watches/warnings/advisories... 
Winter Storm Watch from Sunday afternoon through Monday morning 
for coz033-034. 



Short term...kriederman 
long term...barjenbruch 

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