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Steam & Excursion > Southwest Monsoon expected to start this week

Date: 06/14/22 14:38
Southwest Monsoon expected to start this week
Author: Cumbresfan

A very favorable discussion of an early monsoon from NWS ABQ this morning (Tuesday, June 14):

A stronger than average polar jet, thanks largely to the effects of
a double-dip La Nina, is forecast to result in an upper-level
pattern that was once commonplace during monsoon seasons past but has
been quite elusive over the past 20 years or so. Westerlies strong
enough to force the Four Corners` or monsoon high east of NM. The
southerly flow between this high (forecast to be over the southeast
U.S. late this week into next) and closed upper- level lows over/near
the PACNW will lead to a predicted early start to the North American
Monsoon (NAM) late this week. Daily rounds of afternoon and evening
thunderstorms are expected Thursday through the weekend with southerly
flow at all levels of the atmosphere over NM. Widespread precipitable
water (PWAT) values of an inch or greater move in over the weekend
with locally heavy rainfall most likely Sunday and Monday. Storm
steering flow will send storms to the north and northeast around 20
mph. The good news it that there are no forecasted backdoor front
that will focus storms along the east slopes of the the Sangre de
Cristo mountains. The bad news is that steering flow doesn`t change
much Friday through the weekend with storms likely moving/training
over the same areas day after day. GFS and ECWMF agree that dry
westerlies likely won`t scour out monsoon moisture during the next 10
days. It`s an impressive early start to the NAM to say the least and
well predicted by the sub-seasonal ECWMF-IFS model.


Date: 06/14/22 18:45
Re: Southwest Monsoon expected to start this week
Author: WW

For all in the SW, I hope that this forecast is correct.  While most of the Southwest interior U.S. is very prone to repeated drought, one characteristic that is positive is that the region often has two peak precipitation periods in a normal year late winter-early spring, and the Southwest Monsoon in July-early Sept.  The climatology in Chama, NM is a good example of this.  Unlike California, for example, that normally only has one precipitation peak in winter, the interior SW can, for example, have a dry winter that is partly compensated for by a heavy SW Monsoon season, or vice versa.  While the SW Monsoon is typically not wet enough to make up for streamflow deficits from a dry winter/spring, it can greatly reduce fire danger and replace some soil moisture ahead of the following winter.

The SW Monsoon does bring its own risks, however,  If it is sporadic and not well-supplied with moisture, it can bring dry thunderstorms that put down a lot of lightning, with little rain to put out the wildfires that the lightning starts.  If the SW Monsoon starts out strong then weakens, it can wet the landscape enough to bring on a lot of grasses and forbs that later dry up and enhance fire danger in late summer or fall.  Of course, a too vigorous or wet SW Monsoon can bring torrential thunderstorms that cause at least localized flash floods.  One other feature not always directly related to the SW Monsoon, but a cause of several major floods in the SW (including in narrow gauge country) occurs when the remnants of a Pacific hurricane or tropical storm slam into the Southwest, usually in September or October.  Such tropical storm-related floods ravaged the Silverton Branch in 1909, 1911, and 1970, with less major floods in other years.  There has not been a major tropical storm-caused major flood in SW Colorado since 1970, one of the longest periods on record without such a major flood.

One final note for travelers to the area--while lightning density (number of strikes per square  mile) is much less in southern Colorado and northern New Mexico than it is, for example, in central and eastern Kansas, central and eastern Oklahoma, and northern Texas, the southern Colorado/northern New Mexico area is one of the most lightning-prone in the Rocky Mountain region.  This lightning frequency peaks during the SW Monsoon.  During the SW Monsoon, the climber's rule of "off of the peaks by noon" is a good rule to follow.  Partly because of the terrain and partly because of the amount of people vacationing outdoors, Colorado and New Mexico have some of the highest lightning fatality rates in the U.S. Also, unlike much of the Plains states where lightning density is higher in the late evening and nighttime hours, lightning density in the southern Rockies is highest during the afternoon and early evening hours.  It's always a good idea to regularly check weather forecasts.  A good rule of thumb for higher elevation locales (Durango, Chama, etc.) during SW Monsoon season is to look at the sky early in the day.  If there are ANY clouds present at, say 8-9 AM, that is a good indicator that there is sufficient moisture present in the atmosphere for thunderstorms to develop later in the day, usually early to mid-afternoon. 

By the way, I've studied lightning climatology for decades, and have also photographed a fair amount of lightning.  So, railroad photography is not my only "gig," so to speak.

Edited 1 time(s). Last edit at 06/14/22 18:46 by WW.

Date: 06/15/22 20:09
Re: Southwest Monsoon expected to start this week
Author: ProAmtrak

Excellent detail there WW!

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