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000 
FXUS65 KBOU 280337
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
937 PM MDT Mon Mar 27 2017

.UPDATE...
Issued at 921 PM MDT Mon Mar 27 2017

Upper low moving into the Great Basin at this time with moderate 
mid level qg ascent near the Four Corners. The latest NAM12 run 
drops this down into central AZ by 12z Tue. Plenty of mid and 
upper level moisture moving across the cwa. In addition, mdls 
generate a weak cyclone overnight, advecting low level moisture 
into the northeast plains and I-25 corridor overnight. Have 
included areas of fog in the grids late tonight into Tuesday 
morning, 09-15z. It may linger a little longer over the far 
northeast plains. Will leave the rest of the grids and current 
highlights as is.

&&

.SHORT TERM...(This evening through Tuesday)
Issued at 407 PM MDT Mon Mar 27 2017

Southwest flow aloft will be over the area through tomorrow
morning with the incoming trough continuing to progress SE over
the Great Basin. By 12z tomorrow both the GFS and EC have the low
center progressively further south along with the the Canadian and
NAM solutions. Ahead of the main trough the moisture will continue
to increase over the Western portions of the state and into the
higher terrain bringing rain and snow starting this evening.
Cooler air will move into the mountains along with decent QG lift
that will allow for the snow level to drop down to 8500 ft by
Tuesday morning and continue to lower through Wednesday morning.  
Current forecast accumulations have possible 6 to 12 inches for 
the higher terrain with the most above 7000 ft and over the 
Central mountains south of I-70. By Tuesday morning the models 
show the upper low moving eastward across New Mexico increasing 
SSE flow on the plains turning more easterly by Tuesday afternoon 
and evening. This initial SE flow and increasing lee trough will 
dry slot closer to the foothills delaying the onset of 
precipitation until later in the afternoon/evening Tuesday for 
areas west of I-25. By late Tuesday models have surface winds 
switching to a more NE direction as the low moves south. 
Temperatures will cool tomorrow with highs in the upper 40s to 
lower 50s with some areas dropping into the 30s as precipitation
develops.

.LONG TERM...(Tuesday night through Monday)
Issued at 407 PM MDT Mon Mar 27 2017

Deep moisture and lift will move over Tuesday night into
Wednesday. Models have consolidated on a track that keeps the 
best lift will be across southern Colorado, with moderate lift as 
far north as Interstate 76, and less further north. Blended model 
QPF looks reasonable with amounts of one- half to one inch plus a 
bit more for modest upslope enhancement in the southern foothills.
Big question remains the temperatures. Trend here is also
relatively warm, suggesting a sharp gradient in snowfall at around
6000 feet. It may be cold enough for some snow in Denver Wednesday
morning, but at elevations between 5 and 6 thousand feet there
should be a mix of rain and snow and air and ground temperatures
staying above freezing will limit the impact. Would not be
surprised to see some of these areas get a couple inches of snow
on the grass and maybe a little slush on the roads at rush hour,
but probably not enough for big problems. Higher elevations should
see a period of heavier snow during the late night until about
rush hour. Putting together the precipitation and temperature
patterns leaves us with needing a Winter Storm Watch for areas
west and south of Denver, while other areas are expecting much
less or no snowfall. We may need to look at an advisory for other
mountain areas, but the upslope enhancement should limit the heavy
snow to the watch areas.

This storm will be moving out slowly on Wednesday, with
temperatures recovering fairly well as there is no cold air mass
coming in its wake. There will be a break Thursday with warming
southerly low level flow on the plains, then a little moisture and
instability arrives ahead of the next system that could bring some
light showers to the mountains late Thursday. 

There is reasonable agreement on the next system for Friday and
Saturday. Again the details of placement and temperature will be
important, but it does look like a pretty good chance of
significant precipitation again, and with temperatures similar to
the Tuesday night/Wednesday storm. There are some model runs that
are a few degrees cooler, which could make a difference for
Denver, but there isn't anything to point at a more widespread
significant snow. Consensus now seems to be about the same with
temperatures dropping into the lower 30s in Denver and significant
snow starting somewhere between 5,000 and 6,500 feet. 

Once again, there is no cold air behind the Friday/Saturday system
so we should warm right back up again. There is some divergence in
the models after this. Most keep a string of strong systems moving
through every two or three days. Again, mostly warm, though a few
dew tap some slightly colder air and could pose a threat of snow
in the middle of next week. The latest operational ECMWF is an
outlier building a broader ridge for mild and dry weather in the
early part of next week. In any event, that next system does not
appear to come before next Monday night, so mild and drier in the
meantime.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 921 PM MDT Mon Mar 27 2017

Expect ILS restrictions the rest of this evening with bkn-ovc cigs
050-070 kft agl. mdls indicate e/sely winds overnight will
transition to north/northwest until 09-10z, with fog/stratus
advecting into the terminal areas from the north and west. Could
see IFR cigs/vsbys developing primarily 09-15z window.
Intermittent showers overnight and Tuesday morning, becoming more
prevalent after 21z with MFVR/IFR cigs/vsbys in clouds and pcpn.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Tuesday evening through Wednesday 
morning for COZ036.

Winter Storm Watch from late Tuesday night through Wednesday 
morning for COZ041.

&&

$$

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