Omega block expected across western North America… (H. Michael Mogil, CCM, CBM, DMS)

fig001-omega-block-160922-late-pmComputer models keep suggesting the development of an “omega block” across western Canada by late this week. Such a block can involve a splitting of the overall upper level wind flow by a closed high to the north and a closed low to its south (Fig. 1). Alternatively, the pattern can involve a closed high sandwiched to its west and east by two closed lows. These situations split the flow (even if strong winds are involved), resulting in a slow-moving to stationary upper level pattern in the region of the block. “Omega blocks” are so-named because the northern part of the block resembles the Greek letter, Omega – Ω.

Although such “blocks” often persist for relatively long time periods (due to the aforementioned slow movement and blocking of the overall flow pattern), this particular system will break down by the early to middle part of next week as a deeper trough develops across the eastern U.S.

© 2016 H. Michael Mogil

Originally posted 9/19/16

weather, weather safety/preparedness

Newton weakens; brings flash flood threat to parts of Arizona and New Mexico… (H. Michael Mogil, CCM, CBM, DMS)

Fig001-Newton-storm-track-160907-1500ZAs quickly as Newton formed, it has begun its demise. Moving across rough terrain in northwestern Mexico, Newton has been downgraded to tropical storm status early this Wednesday morning. By this evening, according to National Hurricane Center forecasters, Newton should be in depression status as it moves across southeastern Arizona (Fig. 1).

Fig002-radar-map-160907-1428Z    Fig003-watch-warning-map-early-on-160907Already, Newton has spread tropical moisture northward into Arizona and New Mexico (Fig. 2). With some reports of one to two inches of precipitation already and widespread general rainfall occurring already across southern parts of the two states, flash flood watches have been posted until this evening (Fig. 3). In parts of Maricopa County (west of Tucson), a flood advisory has been posted. According to the Tucson National Weather Service (NWS) office, “as of 6:57 a.m. M.S.T…gauge reports indicated one to 1.5 inches of rain (had fallen) over the past one to two hours…” The rain was linked to a line of heavy showers and thunderstorms moving into the area from the east (within the counter-clockwise circulation of Newton).

With rainfall amounts possibly reaching four inches or more locally, several NWS offices across the flash flood watch area have advised that normally dry arroyos (dry stream beds) will likely see rising water as the day progresses. And with the rainfall expected to continue all day, as is always the case with arroyos and low water roadway crossings in dry wash areas, the NWS has urged drivers to be especially cautious and not drive into any flooded areas or any areas with moving water. Conditions will be more treacherous in mountainous areas (due to merging runoff from small watershed areas) and downstream from any burn scar areas (linked to recent forest or grassland fires). Scar areas are often more impervious and lack vegetation that can lessen flooding dangers.

Once Newton moves through the area overnight, only slight chances of shower and thunderstorm activity are expected for the remainder of the week.

© 2016 H. Michael Mogil

Originally posted 9/7/16

weather, weather safety/preparedness

Tropical Storm Hermine is now getting weird (H. Michael Mogil, CCM, CBM, DMS)

Fig001-NHC-Hermine-track-fcst-from-160903-5amEDTTake a look at today’s early morning (Sept. 3, 2016) Hermine forecast track from the National Hurricane Center – NHC (Fig. 1) and you’ll see several things appear to be, well, just plain weird. However, there is some good rationale behind each of these.

First, Hermine is expected to slow down and possibly try to make a cyclonic (counterclockwise in the Northern Hemisphere) loop off the New Jersey coast early next week. This is linked to Hermine’s interaction with a developing, middle latitude, cold core, cut-off upper level low-pressure system.

Next, the 72-hour forecast indicates that Hermine will transition into a middle-latitude or extra-tropical hurricane. This, too, is linked to interactions between the upper low and Hermine. It is also tied to Hermine exiting the Carolina’s and moving back over relatively warm waters. The storm will become a hybrid with some attributes of a hurricane (wind speed core near the storm’s center) and some associated with a winter-like storm (colder central core temperatures).

Due to proximity to land, and with lessons learned from Storm Sandy in 2012, NHC will continue to issue advisories on Hermine, even after the storm becomes extra-tropical. This is to better keep public attention on hurricane dangers.

Finally, hurricanes do not usually slow down and spin within a relatively confined geographical area, once they recurve to the northeast. However, the “Perfect Storm” (a.k.a., the Halloween Storm of 1991) and Storm Sandy in 2012 (see synoptic maps animation) are among storms that have undergone a similar type of tropical/middle-latitude interaction in which a middle-latitude, upper level low “captures” the hurricane circulation.

Hermine is not done wreaking her havoc on the U.S. East Coast. She has already produced a significant storm surge and associated coastal erosion in Florida’s Big Bend region – mainly Taylor County (thanks to the storm’s track and the shape of the coastline immediately to the southeast of the storm’s path); excessive rainfall along Florida’s west coast, including nearly 20 inches in the Tampa area (Pinellas County); and widespread wind damage to homes, trees and power lines (with significant power outages – in the hundreds of thousands).

Fig002-Atlantic-City-5-day-fcst-TS-conditions-160903Early Saturday morning, tropical storm watches and warnings, storm surge alerts, forecasts of additional excessive rainfall and tornado watches cover at least parts of coastal locales from the Carolinas into southern New England. However, the best measure of the overall, long-term, potential impacts is the five-day graphic forecast for Atlantic City, New Jersey (Fig. 2). Tropical storm conditions (including pounding surf and storm surge) will likely cause serious coastal damage and erosion.

© 2016 H. Michael Mogil

Originally posted 9/3/16