May Heat Ahead…Forecast Numbers Going Up

May 18, 2017

6pm Thursday

Today was sure a nice day after 7 days of rain showers.  Other than a sprinkle or two late this afternoon, we saw partly cloudy skies and much warmer temps.  This is about as normal as it gets in the latter half of May

High pressure is strengthening over the Pacific Northwest as the cold upper-level trough has moved off to the east.  In general high pressure will dominate our weather for the next week.  Temperatures climb tomorrow with very weak onshore flow and a warmer atmosphere overhead.

Saturday will be cooler with even a chance of a shower or drizzle through midday.  That’s because a weak system rides through the building high pressure, allowing a surge of cool marine air inland.  That marine layer will be quite thick Saturday…up to about 4,000′ or so.  When it does that the possibility for a shower or drizzle exists.  The ECMWF is sure seeing this, although less than .10″ in any lowland location

Sunday through Tuesday a big upper-level ridge builds with 500 millibar heights above 585 dm.  That is a summertime ridge and air overhead will be warm like summer.

Models are in excellent agreement that 850 millibar temperatures (the temperature around 5,000′ elevation) will rise to around +15 (celsius) Sunday, +19 Monday, and +20-21 on Tuesday.  That’s just plain hot overhead.  On Monday and Tuesday easterly flow at that elevation lowers pressure west of the Cascades.  It appears all day Monday through early Tuesday what we call a “thermal trough” will be located quite close to the coastline.  A thermal trough is a very warm area of low pressure that forms in the lee of the Cascades/Coast Ranges.  In this case, with it getting out along the coastline, temperatures should skyrocket at the beaches just like in the valleys.  Most likely that happens on Monday; that IS the one day that should be a scorcher on the beaches…80-90 is possible out there.

Here in the metro area, if we combine May sunshine, offshore wind flow (both Monday & Tuesday), and those 850mb temps?  We get a high temperature somewhere between 92-98 based on past setups in May just like this.  Highest temps are likely on Tuesday assuming no onshore flow begins.  As a result I raised 7 Day forecast high temps a could degrees again today.

TO SUMMARIZE

  • NO RAIN THE NEXT 7 DAYS, UNLESS WE GET A SPRINKLE OR DRIZZLE SATURDAY AM
  • COMFORTABLE TEMPERATURES THROUGH SUNDAY
  • MONDAY & TUESDAY WILL BE HOT DAYS, ESPECIALLY FOR MAY.  92-98 IN THE METRO AREA
  • IT APPEARS THIS WILL BE OUR WARMEST MAY WEATHER IN 9 YEARS
  • EXPECT DRY HEAT THIS TIME WITH THE DRY NORTH & EASTERLY WIND
  • SUNDAY & MONDAY AT THE COAST?  75-90 DEGREES, WARMEST MONDAY

 

So when might it rain again?  Some ensemble members of some models imply a weak upper-level low could bring showers/clouds Thursday and beyond next week.  That said, more members are remaining dry through Memorial Day Weekend than wet.  Only 5 of the 21 GEFS ensemble members give PDX .10″ or more rainfall by the end of that holiday weekend.  Each of the horizontal lines below represents 1 of those 21 members.  Time goes from now on the left to the 3rd of June on the right.

The ECMWF ensembles are similar…11 of 51 members produce .10″  rain in the next 10 days.   Only 4 have .50″ or more.

Basically we MIGHT get a few showers as we head toward/into Memorial Day weekend, but as of now there’s no reason to believe a long spell of cold and showery weather is coming back before the end of the month.  There’s also no sign that hot weather continues beyond Tuesday.

Chief Meteorologist Mark Nelsen