Heavy Rain in Texas as Progressive Pattern Leads to Increased Forecast Uncertainty

By: Steve Gregory , 8:50PM,GMT on October 22,2015

THURSDAY: 22-OCT-15 / 4:00 PM CDT
(NEXT UPDATE: MONDAY - OCT 26)

FLASH FLOOD THREAT TO TEXAS AS HIGHLY PROGRESSIVE PATTERN CONTINUES TO INTENSIFY

With upper level heights rising across the eastern US, much warmer conditions swept across the eastern US this week while the high amplitude TROF in the west has moved into the Rocky Mountain region spreading locally heavy rains ahead of it from the southern Rockies across West Texas.

Between the approaching high level TROF and a surge of deep, tropical moisture laden air associated with Hurricane PATRICIA now approaching the southwest Mexican coast, there is a (continued) growing threat of extreme rainfall and flash flooding in portions of Texas as we move through the weekend. The TROF and developing tropical flow will first bring a period of heavy rain to central and western TX - with the remnants of Hurricane PATRICIA then expected to bring a second and stronger rain event to central and east TX during the weekend. In total, widespread 2-4 inches of rain – with localized 5-Day totals over 10” - appears likely. These rain producing systems will then advance northeastward towards the Great Lakes region and East/Northeast US next week. The exact path of the heaviest rainfall is fairly uncertain due to the broader scale uncertainties that continue to plague the modeling programs.

Looking towards Week 2 – and based on the most recent GFS model run this morning – a continuation of rapidly moving short wave TROFs from the Pacific across the nation is expected to lead to a fairly strong and fast moving TROF which will then deepen rapidly as it reaches the Great Lakes/Midwest region southward into the SE US by the end of next week. This should bring another surge of cold air behind it. However, since the TROF is also forecast to continue moving eastward after deepening, the cold air outbreak is unlikely to last more than a few days.

At the same that the TROF is deepening in the east, the next upstream TROF will be developing along the west coast, and this TROF is forecast to generate another upper Low formation over southern California, along with significant rainfall which will then move eastward across the SW into Texas during Week 2. It should be emphasized that with such a fast moving pattern, it is far too soon to know with any real certainty if the system will actually produce significant rainfall in SO-CAL since it’s highly dependent on the amount of moisture that will be advected into the region from both the NORPAC to the west - and the tropical Pacific region - to the south.

EL NIÑO AND EMERGING MJO SIGNAL
The latest tropical region analysis shows an emerging and strengthening MJO signal in the far western portions of the Indian Ocean – with all global models now forecasting the MJO to continue intensifying over the next 2 weeks while remaining quasi-stationary through the next 7 days before beginning to propagate slowly towards the east. The longer range ECMWF bi-weekly MJO forecast (not shown) calls for the signal to reach the far western Pacific during mid-November - and if this prediction verifies - the MJO will begin to impact the ENSO region by late November into December – with its affects dependent on the structural makeup of the oscillation and it’s actual rate of forward motion.

Meanwhile, SST anomalies in the benchmark Niño 3.4 region remains near +2.5˚C above normal (although the most recent daily analysis shows a significant decline to +2.2˚C which is likely attributable to extensive cloudiness and shower activity over the past week associated with tropical cyclone activity near the region.

In addition, low level wind analysis for the 850mb (5,000 Ft) level indicates there was a fairly strong Westerly Wind Burst (WWB) last week with continuing westerly wind anomalies across much of the west central / central Pacific – and a forecast for yet another Westerly Wind Burst (WWB) starting this weekend in the 3.4 region to the east of the Dateline.

While not being explicitly depicted in the latest CFSv2 forecast, it’s reasonable to expect SSTA’s to rebound back to +2.5˚C early next week before rising close further in 6-8 days in response to the most recent WWB. In addition, if the forecast WWB in the ENSO region next week materializes – it’s also reasonable to expect SST anomalies to increase further during early NOV before stabilizing thru mid-month. It’s after mid-November when the impact of the developing MJO can be expected to have a material effect on both the SSTA’s in the ENSO regions and the atmospheric flow pattern due to various tropical forcing mechanisms.

ALASKAN WEATHER
There’s been little change to the overall upper air pattern in the Alaskan region, with a mean upper TROF extending from Siberia/Kamchatka southeastward to the southern GOA with ridging over NW Canada. This pattern continues to bring an unseasonably mild flow across much of the state. A typical pattern of storm formations over the Aleutians which then move into the GOA will continue to bring periodic Precip to the southern coastal region, with much above normal Temps continuing for the next 6-10 days. There is some indication that the mean upper level TROF location will shift towards the east in 10-14 days – allowing significantly colder air to begin moving into the state during early NOV.




Fig 1: 15-Day 250mb ( ≈35,000 Ft) Wind Forecast Loop The Polar Jetstream (which typically is found around the 250mb level on average) is strengthening as it remains highly progressive. I have drawn the mean position of the primary TROF’s (red dashed lines) and upper level High Pressure systems at the start and very end of the forecast period. Note that by the end of the forecast period, the pattern is expected to become significantly more zonal (west-to-east orientation of the jet stream(s)).


Fig 2: Water Vapor Satellite Image of the EPAC This image from a few days ago highlights the extensive cloudiness and Shower/T-Storm activity across the ENSO 3.4 region during the past 5 days – primarily associated with the still quite impressive ITCZ (Inter-Tropical Convergence Zone) and persistent tropical cyclone activity. It appears that the prolonged period of overcast conditions has allowed SST anomalies in the 3.4 region to drop off to near +2.2˚C– PRIMARILY due to cooling of SST's over the western portion of ENSO 3.4 region (west of 150˚W longitude).


Fig 3: Observed and Forecast 850mb (~5,000 Ft) Wind Anomaly Hovmöller Diagram for the Tropical Pacific Following the strong WWB near and to the West of the Dateline 7-14 days ago, another WWB is expected to develop to the east of the Dateline over the weekend. The 850mb westerly wind anomalies have begun to work their way to the surface again, and this should help to intensify the latest Oceanic Kelvin Wave in the EPAC. This in turn should lead to at least some additional warming of SST’s.


Fig 4: MJO Analysis and Global Model Forecasts A new MJO cycle appears to be evolving, with a strengthening signal now developing near east Africa and the far western Indian Ocean – as has been forecast by both the GFS and ECMWF global models for the past week. With both the operational and ensemble models continuing to forecast a moderate to strong signal in the Indian Ocean region during the next 1 to 2 week. The signal is forecast to be essentially stationary for the next 6-10 days before beginning to propagate eastward. If this forecast verifies, it will have a material impact on El Niño starting in about a month.


Fig 5: Temperature ANOMALY forecast for Week 1 is based STRICTLY on the MOS data forecasts from the todays 12Z operational GFS model run with only minor adjustments towards the raw model data points for Days 6 and 8. Temperatures will average well above normal across most of the nation during Week 1 - with the possible exception of Texas where prodigious rainfall during the next 5 days will tend to keep daytime Temps below average. Overall Confidence in the forecast anomaly Pattern and Magnitude is near average for this time of year, with a reading of ‘3’ on a Scale from 1 to 5.


Fig 6: The Week 2 Temperature ANOMALY forecast is based on the 12Z run of the GFS (80%) integrated with the 12Z GFS Ensemble (10%) and the 00Z EURO model Ensemble (10%) using the projected pattern along with explicit surface and 850mb (~5,000 FT) Temp forecasts. Some Temps are adjusted for known or expected anomalous thermal patterns & projected storm system passages.ASSUMING the amplification of a TROF in the eastern US late next week, colder air can be expected to overspread the eastern states during Week 2 as well above normal Temps continue over the western two thirds of the nation. Model consistency has increased again during the last couple days, so Confidence in the anomaly Pattern and magnitude – especially over the eastern and much of the far western US - has now fallen below average again, with readings of ‘2’ on a Scale of 1 to 5.


Fig 7: GFS 10-Day Total Precipitation Loop (for mounts greater than 0.25” only) Locally heavy to extreme rainfall is likely in TX during the next 5-days – with a swath of heavy rainfall spreading northeastward into the Great Lakes and NE US during next week. As the next TROF develops near the west coast in 7-10 days, significant Precip will reach into northern CA by the end of the forecast period – with Precip reaching into SOCAL during the day 10-14 period (not shown).


Fig 8: Temperature Anomaly Forecast for Alaska during Week 1 Above to much above normal Temps will persist for the next 6-10 days, with anomalies expected to fall off during Week 2.

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Steve