The biggest weather story across the United States this weekend will be the developing western Gulf system which if named, would be named Jerry but this is not the single most important aspect but more it’s the indirect effects this system may have. A strong front will cross Texas over the next 72 hours, slowing as it does so and with developing ‘Jerry’ down over the Gulf, a LOT of moisture is projected to lift north bringing a real soaking to Texas and over the next several days, right across the Gulf Coast and across the southern states extending into the southern Appalachians.

When your talking heavy and persistent rain in an area plagued by drought, your always concerned about the risk of flash flooding, especially when the terrain is flat.

Latest QPF chart for the next 7 days!

This rain would of course be very welcome given the current deficits across Texas.

How Did The Historic Colorado Floods Come About?

Converging weather systems, steep mountains terrain and even the recent wildfires all played a role is creating the ‘perfect rainstorm’ over Colorado.

A blocking high pressure system anchored over the eastern Plains (to the east) and an upper low positioned over western Utah (to the west) produced a SSE flow over Colorado. The trouble is that to the south, exactly where that flow was coming from, was not one but two moisture rich tropical cyclones spinning on both side of Mexico.

While it rained, rained and rained over Mexico. The two upper level wheels spinning on both sides of Colorado, drew this Pacific and Gulf of Mexico origin moisture north and straight  up against the Front Range of the Rockies and we know what happens when moist air is forced up against mountains, it’s forced to rise, cool and condense, producing yet more moisture. What was a two-f0ld problem was the shape of terrain with Boulder nestled at the bottom of a canyon, so heavy precipitation was always going to fall not only over Boulder itself but up in the higher elevations, in even heavier fashion. Where does all that rain water go? Downhill and into Boulder. The second of the two fold issue here is the recent wildfire which stripped a lot of vegetation off the hills, so the water came down at a faster rate than normal.

Here’s a graphic showing the situation.

While Colorado may be drying out under high pressure, all that water continues to flow down hill. A ton of flood water is flowing into the rivers which head into the high plains. The South Platte River is under the gun and is expected to burst it’s banks. Major flooding in Nebraska will likely make headlines in coming days.

West Turns Wetter, Colder With First High Sierra Snow Next Week

By late next week both ECMWF and GFS show a deepening trough over the West indicative of a negative PNA signal and even the far reaching influence of Super Typhoon Usagi. While it gets colder, wetter and with the first mountain snows over California and Nevada. a ridge will in turn pump heat into the Midwest but there’s confliction between ECM and GFS about how far east the ridge gets.

Here’s the 162 hour ECM at 500mb.

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Note the difference between the ECM with it’s shorter wavelength and the GFS (below) with it’s longer or broader wavelength over the West. The shorter wavelength idea from the ECM shows a more amplified pattern with the ridge further back over the Plains and Ohio Valley and therefore the Northeast ISN’T under the ridge and SW flow but under an upper low sitting overhead making for a very different few days of weather (cooler, cloudier and wet) compared to the GFS which drives the heat all the way to New York and Boston.

Here’s the 162 hour GFS

As for snow, it shows more coming to the Rockies from Colorado up to western Montana extending into Canada and also over the northern Sierra Range with the possible first snow of the season around Lake Tahoe.

Surprisingly the GFS is not on board with snow only confined to the high ground around Yellowstone Park. Given the trough it has diving down, I’m not buying this at all. Snow should be more widespread above 6 or 7,000ft and I expect it to snow all the way down to Lake Tahoe.

The Typhoon Effect On The United States

Tropical Storm Man-Yi brushed Japan with flooding rains and strong winds before heading north impacting South Korea en-route to the north Pacific. This has lead to this weekend’s eastern NA trough following a somewhat deeper trough at the start of this week which produced the coldest temps since May from Ontario and Quebec down to the Gulf Coast states.

Though not particularly significant, here’s the trough dropping into the East this weekend, likely as a result of recurving Man-Yi.

The system crossing Canada that’s responsible for carving out the trough, should produce snow over Ontario.

As well as Quebec as it brings wet, windy weather to the Maritimes late weekend.

Here is much stronger and more concerning SUPER TYPHOON USAGI which is heading WSW towards the southern tip of Taiwan and more worryingly, Hong Kong. This all you can see is not recurving and so one would expect a ridge over the Midwest and East mid to late next week… Fits what’s just been shown above right?

Here’s the effect the typhoon should have, not solely but at least should have an effect.. to the late next week pattern over the US.

Check out the 8-14 day temperatures

Hope to get another post up later today on the fight in long range models over October and further down the road. +PNA, -NAO/AO coming up, still hints strongly at colder times in the Midwest and Eastern US towards early October.

PNA is more uncertain..

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Tags: Boulder flood, colorado flooding, Texas flood threat, Typhoon, typhoon recurve, USA weather