479 
FXUS65 KBOU 210353
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
953 PM MDT Mon May 20 2019

.UPDATE...
Issued at 953 PM MDT Mon May 20 2019

Heaviest snowfall is expected to continue through the midnight 
hour per latest radar/satellite data, and latest totals and 
snowfall rates prompted the upgrade to an Advisory in the 
immediate Denver area (2-6", heaviest on the south/east sides of 
town), and an upgrade to a Winter Storm Warning over the Palmer 
Divide where 7 to 15 inch totals likely. We may see a decrease in 
intensity after midnight with some drying noted to the south, but 
there's still a lot of forcing later tonight through early Tuesday
morning for additional accumulation in that time frame. Main
impact for Denver will be broken tree limbs and scattered power
outages in the areas that see the heavier totals, and some
slush/snow on roads during heavier snow periods. More significant
travel troubles over the Palmer Divide with much heavier totals of
heavy, wet snow. 

UPDATE Issued at 845 PM MDT Mon May 20 2019

Radar has been showing several bands of precipitation passing 
north across the forecast areas, including I-70 from Bennett to 
Strasburg to Byers. In this area web cameras are indicating snow 
accumulating on grassy surfaces in locations just east of Denver,
and has been heavy at times. Satellite and radar depict 
stratiform snow funneling up the east side of the Denver metro 
area tonight. For this reason the Winter Weather Advisory has 
been extended into eastern portions of Adams and Arapahoe Counties
with snow accumulations of three to seven inches. Temperatures
have trended colder as well, so we will add additional snowfall 
to the I-25 corridor from Denver north to Fort Collins. 

The upper level low is progressing towards the four corners 
region. Satellite imagery indicates a jet speed max sliding north 
up the eastern side of Colorado creating strongly diffluent flow 
and surface pressure falls. This will allow for more lift and 
heavier snow. The best time period for snowfall across the 
mountains, foothills, and Palmer Divide will be through the 
overnight period. The RAP and HRRR models have the moisture 
decreasing through the early morning, while the GFS and HiRes Euro
keep precipitation through Tuesday. Basically, the I-25 Corridor 
and nearby adjacent plains and Palmer Divide area will have to be 
monitored closely to see how these waves of precipitation come to 
fruition, because at the intense snowfall rates and high water 
content, it won't take long to cause some problems with more 
snow/slush on roads, and possibility of broken tree limbs and 
scattered power outages where heavier snow falls. 

See below for current Winter Storm Warning and Winter Weather 
Advisory highlights.

&&

.SHORT TERM...(This evening through Tuesday)
Issued at 238 PM MDT Mon May 20 2019

The strong upper low which is currently approaching the 4 corners 
region this afternoon is providing extensive strong mid and upper 
level ascent and diffluence over eastern CO. A narrow dry slot over 
northern NM and the San Luis Valley has allowed for some surface 
based convection to fire in area of partly cloudy skies and 
slightly higher cape. This is being drawn northward into the 
central mtns and NE CO area early this afternoon. Will expect 
northward moving showers to continue into the evening along with 
batches of weak to moderate thunderstorms passing across the 
higher terrain and onto the Urban Corridor and plains. Can't rule 
out a few stronger storms containing brief moderate to heavy rain 
and small hail as they move into a supportive shear environment 
with a weak Denver cyclone currently in progress. Another area of
convection and lift, closer the upper low near Flagstaff AZ is 
still behind this initial convective feature and will likely be 
the snow maker and for the mountains and a precip producer for the
lower elevations once it arrives over the central CO. The stable 
drizzle-like sounding across the plains from this morning 
destabilizes  to a degree in the mid and upper level for late and
afternoon and tonight, however remains somewhat stable near the 
surface due to persistent and fairly deep northeast flow at the 
surface. Forecast cross sections and sounding become near 
saturated from the surface thru 400mb with lapse rates holding in 
the 6.5-7.5 C/km range thru the 03z to 12z Tuesday timeframe over 
the mountains. Can still expect warning criteria snowfall for the 
mountains as we typically see convective generated snowfall this 
time of year and especially under this type of synoptic 
environment. Snowfall totals of 15 inches plus will not be out of
the question due to persistent showers and possible banding, 
especially on south and southwest facing aspects. For the lower 
elevations, slow levels still expected to fall near the 
5000-5200ft elev as wet bulb 0C levels have come in lower on 
current runs. Will add Palmer Divide to the Winter Wx Advisory 
thru Noon Tue as 5-10 inches may be possible especially across 
the higher western portions of the ridge. 

Looking at Tuesday, will be watching for the timing and extent of 
drier surface downslope flow pulled off the higher terrain and onto 
the lower elevations of the NE plains. By 12z Tuesday, the upper 
closed low near the OK panhandle begins a fairly rapid exit out into 
NW KS by 18-21z. This will likely bring the beginning of the end to 
the best precip across the lower elevations as drying NW downslope 
flow fills in behind ejecting low. As upper low exits the region, 
NAM and GFS swing another trof axis across the CWA from the north 
for late afternoon Tue with another brief round of reinforcing cold 
air aloft. With this last push of cold air advection aloft this
may keep the snow or snow showers going into early afternoon 
Tuesday across the mountains and higher foothills with weak 
convective rain mixed with snow showers thru afternoon across the
lower elevations. By late afternoon into early evening Tue, 
strong QG subsidence begins to work into southern and central 
Colorado with rapid lowering of RHs across the CWA.

.LONG TERM...(Tuesday night through Monday)
Issued at 238 PM MDT Mon May 20 2019

For Tuesday night into Wednesday, the upper level low will be moving 
to the northeast into central NE. A deep surface low will be over 
western KS to south-central NE. The wrap around provided by the 
cyclonic flow of the surface circulation will keep strong NW flow 
moving into the region with high moisture. This will allow for 
continued showers over the plains into Wednesday morning and snow in 
the higher elevations. Warm air advection Tuesday will change over 
cold air advection as winds in the mid levels turn more to the 
northwest. Models are showing enough of a cool down in the mid and 
lower levels that some snow could mix in with the rain overnight 
into early Wednesday across the plains, mainly North near the 
Wyoming border and the northern foothills. Low temperatures for 
Tuesday night will be hovering around the freezing mark with values 
from 31 to 34 across the plains. The surface low will continue to 
track to the NNE through the day Wednesday pulling the energy and 
moisture north leaving CO in SW flow aloft. Conditions will dry over 
the plains with increasing subsidence but there is a slight chance 
of storms over the higher terrain with lingering moisture and some 
instability present. Highs on Wednesday will rebound slightly with 
temperatures in the 50s. 

For Thursday, the next upper level disturbance that will affect 
Colorado will be over Utah and is currently forecasted to push North 
into Wyoming and southern Idaho. At the surface, a deepening low 
pressure system over the southern portions of the state will move to 
the NNE pulling in some moisture in the cyclonic SSE flow. This 
could help to provide high enough dewpoints across the I-76 corridor 
for a chance of showers and thunderstorms Thursday afternoon. 
Elsewhere, lower chances of convection are possible with a chance 
across the higher terrain. Temperatures will continue to increase 
with highs projected to be in the lower 60s across the plains and 
40s to 50s in the mountains. 

Friday and into the weekend, the upper low will continue to push 
North into Canada leaving Colorado in SW flow. Increasing subsidence 
from the SW with WAA in the mid levels will help to stabilize the 
plains and bring mostly sunny conditions. Temperatures will be 
closer to seasonal normals with highs back in the upper 60s to lower 
70s with just a slight chance of convection over the mountains. This 
warmer and drier pattern will continue into the weekend with highs 
back into the 70s under just a slight chance of storms for the 
mountains. 

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 845 PM MDT Mon May 20 2019

IFR conditions with snow will remain in the forecast through early
Tuesday morning. Accumulations of 2-5 inches in the grass will be
possible, along with some slush on runways. The heavier amounts
would favor KAPA and KDEN.  Ceilings are expected to slowly lift 
late Tuesday morning, with more of a mix of rain and snow toward
16Z-18Z. MVFR conditions should prevail thereafter with scattered
showers around and only slight improvement of ceilings. 


&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until noon MDT Tuesday for COZ041.

Winter Weather Advisory until 9 AM MDT Tuesday for COZ040.

Winter Weather Advisory until noon MDT Tuesday for COZ045.

Winter Weather Advisory until midnight MDT Tuesday night for 
COZ036.

Winter Storm Warning until midnight MDT Tuesday night for COZ031-
033>035.

&&

$$

UPDATE...Barjenbruch
SHORT TERM...Fredin
LONG TERM...Bowen
AVIATION...Barjenbruch