Hi all,

Hello from the SPC. This discussion... and getting drenched when ~5 in of rain fell in a little over 2 h in Norman last evening... motivated me to use the NOAA/ESRL/PSD interactive web link to construct selected mean and anomaly maps for the NH for 4-18 May 2015. This period corresponds to the start of persistent rains in OK through the date of the latest data available online. The following images are attached:

1. NH mean/anomaly 850/300 hPa geopotential heights
2. NH mean/anomaly 850 hPa temperatures
3. Precipitable water (PW) anomalies

Key take-home points from these maps include:

1.  Anomalously strong 300 hPa jet streams prevail over the western North Pacific and the central and eastern North Atlantic
2.  The exit region of the western Pacific jet stream lies upstream of a strong "high-above-low” block over eastern North America 
3.  A “low-above-high” block is centered over southeastern Canada upstream of a deep trough over the central and eastern North Atlantic
4.  A strong 300 hPa ridge is centered over northwestern Africa and the Iberian Peninsula (recall my earlier post about the heat wave in Spain)
5.  The NH is characterized by relatively high-amplitude 300 hPa flow in middle and high latitudes
6.  The 300 hPa flow is characterized by strongly diffluent flow east of the Dateline and high latitude anticyclonic wave breaking (AWB) over Alaska
7.  The 850 hPa flow pattern is characterized by an anomalously strong southerly flow east of the Dateline from the subtropics to Alaska
8.  This anomalous 850 hPa southerly flow contributed to the poleward transport of warm, moist air and Alaskan ridge building
9.  Anomalous easterly flow at 850 hPa over the Pacific Northwest and southwestern Canada contributed to warm and dry conditions there as noted by Cliff Mass
10. Anomalously strong and poleward-shifted Bermuda high and an anomalous deep trough over the Southwest at 850 hPa contributed to an anomalously strong southerly flow east of the Rockies
11. The anomalous 850 hPa temperature pattern favored (warm to the east; cold to the west) favored anomalous warm-air advection with upslope flow east of the Rockies
12. Anomalously high 850 hPa temperatures over northwestern Africa and southwestern Europe link to the anticyclonic shear side of the anomalously strong eastern Atlantic jet stream
13. Anomalously high PW values over the tropical and subtropical Pacific link to El Nino conditions and a deep poleward moisture transport east of the Rockies

Some science issues:

1. What role did recurving western Pacific TCs play in anchoring an anomalously strong jet stream west of the Dateline on synoptic time scales?
2. What role did El Nino conditions play in enabling early season TC formation over the western Pacific on sub seasonal time scales?
3. What role did high-latitude AWB over Alaska downstream of the strong western Pacific jet stream play in anchoring a persistent “high-over-low” block over western North America
4. What role did anomalous and persistent diabatic heating east of the Rockies play in the formation and persistence of the “low-overd’high” block over eastern North America?
5.  What role did the El Nino-driven low-latitude anomalously strong STJ play in enabling anomalously high values of tropical PW to be transported poleward east of the Rockies?
6. What are the relative contributions of tropical and midlatitude forcing on different time and space scales to the maintenance of the aforementioned remarkable observed spring flow pattern?

Lance