Heavy Rain To Impact The Region Mid Week

Hello everyone!

Special evening update tonight to discuss an upcoming heavy rain/strong wind event slated for the afternoon hours Wednesday into the morning Thursday. If you want to skip the explainers, scroll to the bottom where I have a summary of the impacts and timing of the storm. Lets dig right in.

Setup

By looking at the satellite imagery this evening, you can see all the players on the field so to speak most notably ex-Patricia in the Gulf of Mexico and a Pacific storm near the Washington coast.

IR Satellite Loop Showing Ex Patricia In The Gulf Of Mexico And A North Pacific Storm Over The Pacific NW. Credit: NOAA

Ex Patricia will move east and meander through the Gulf States causing flooding issues for LA and MS in the next few days. From there, the system and all its moisture moves NE. The Pacific storm moves over the Rockies and redevelops in the Canadian Plains. They look to combine over the Great Lakes developing into a powerful low by Wednesday.

12Z GFS Model Showing Ex Patricia And The North Pacific Low Preparing To Combine Early Wednesday. Image Credit: Accuweather Additions By Me.
12Z GFS Model Showing Ex Patricia And The North Pacific Low Preparing To Combine Early Wednesday. Image Credit: Accuweather Additions By Me

By Wednesday the two will have combined into a strong low over the Great Lakes and a strong cold front will be heading our way. Tying together Patricia’s moisture and the energy from the northern Pacific storm.

Impacts

Rain

12Z GFS Showing Tropical Moisture Streaming Northward From Ex Patricia. Credit: Weatherbell, Additions By Me
12Z GFS Showing Tropical Moisture Streaming Northward From Ex Patricia. Credit: Weatherbell, Additions By Me

With one of the main players being Ex-Patricia in this storm, there will be a lot of available moisture. The map above is Precipitable Water for late Wednesday night showing tropical moisture streaming northward ahead of the front signaling the atmosphere is ripe for heavy rain. Consider the sponge saturated. Now all we need is something to wring it out.

12Z GFS Verical Velocity Showing Intense Upward Motion Wednesday Evening. Image Credit: Accuweather, Additions By Me
12Z GFS Verical Velocity Showing Intense Upward Motion Wednesday Evening. Image Credit: Accuweather, Additions By Me

If you’ve followed the blog for a while, you’ll know vertical velocity is one of my favorite maps to show during heavy precip events. For those who may be unfamiliar with this map, it can be boiled down to how fast air is rising. The faster air rises, the faster it cools/condenses and thus the faster the moisture can be turned into rain. Basically: the more vertical velocity (upward motion), the heavier the precip. This force is what wrings out the sponge that the moisture from Ex-Patricia brought. The above image shows intense upward motion along the coastal plain Wednesday evening that will spread NE through the night indicating a thorough soaking for all. In terms of final amounts, a general 1.5-2.5″ is likely with some localized amounts of 3″+ generally on the south facing slopes of the Whites where upsloping will be strongest.

Wind

12Z GFS Showing Strong Winds Just Above The Ground ‘Available’ To Be Mixed To The Surface. Image Credit: Accuweather, Additions By Me

With intense low pressure rapidly intensifying to our west, it’s no surprise winds look to be an issue with this storm. Shown to the right is the GFS Low Level Jet parameter which gives an indication of what the winds are a few thousand feet above the surface. Given the proper conditions, some part of these winds can be ‘mixed’ down to the surface in heavy rain bands. The downdrafts associated with these heavy rain bands acts kind of like a mirror deflecting some of the winds aloft towards the surface. Not all of the winds make it down.  While this model indicates near hurricane force winds aloft, winds at the surface are unlikely to be that strong. Expect winds at the surface to range from 20-35mph with gusts approaching 50mph. That’s still high enough to cause some issues with scattered power outages and tree branch casualties. Winds will be out of the south/southeast for Wednesday and early Thursday before turning NW later in the day Thursday.

Coastal Flooding

Wave Watch III Model Showing Building Seas Offshore Wednesday Night Resulting In Some Minor Coastal Flooding During The Wednesday Midday And Wednesday Night High Tides. Image Credit: Accuweather, Additions By Me
Wave Watch III Model Showing Building Seas Offshore Wednesday Night Resulting In Some Minor Coastal Flooding During The Wednesday Midday And Wednesday Night High Tides. Image Credit: Accuweather, Additions By Me

With strong SE winds for a decent amount of time and for an exceptionally long distance over the Atlantic combined with astronomical high tides this week will create the risk for some coastal flooding. This is by no means a textbook coastal flooding event and only minor impacts are expected for most but some beach erosion and minor splashover are likely. The two high tides to watch will be Wednesday midday and Wednesday night but due to the slow evolution of the SE winds Wednesday, there is unlikely to be a prolonged period of strong winds and thus coastal flooding looks to remain a relatively small threat.

Summary

Rain will move into the area Wednesday afternoon continuing through Wednesday night and extending into Thursday morning. Rain will be heavy at times Wednesday night adding up to 1.5-2.5″ for most with some 3″+ totals on the south facing slopes of the mountains. Winds will also be gusty with 20-35 mph winds out of the south/southeast gusting near 50mph Wednesday night.

I’ll be back tomorrow morning with more on tomorrow’s weather and I’ll have another update on this rain event tomorrow evening.

-Jack

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